Download Photofragmentation-laser induced fluorescence: a

Photofragmentation-laser induced fluorescence: a new
method for detecting atmospheric trace gases
M. 0. Rodgers, K. Asai, and D. D. Davis
A new method for the in situ detection of nonfluorescing molecular species is proposed: photofragmentation-laser induced fluorescence (PF-LIF). In this approach, the species to be detected is first laser photolyzed at a wavelength X1, producing one or more vibrationally excited photofragments. Before vibrational
relaxation occurs, one of these photofragments is pumped into a bonding excited state by a second laser
pulse centered at wavelength X2. Fluorescence is sampled at a wavelength X3, where X3 < \2 and X. This
pumping configuration thus permits massive discrimination against Rayleigh and Raman scattering as well
as white noise fluorescence from the laser wavelengths Xi and X2 . The technique should be both highly sensitive and selective for numerous atmospheric trace gases. Specific sampling schemes for detecting NO2 ,
NO 3 , and HNO2 are proposed. Various noise sources and chemical interferences are discussed. Specific
calculations that estimate the sensitivity of the PF-LIF system for detecting NO2 , NO3 , and HNO 2 are
given.
1. Laser Induced Fluorescence (LIF) Technique
Laser induced fluorescence, as a highly sensitive and
selective technique for the detection of gas phase molecules, dates back to the early work of Sukurai and
Broida.1 Since that effort, numerous spectroscopic
applications of the LIF technique, particularly using
tunable dye lasers, have been reported in the literature.2
The basic approach of the LIF technique is illustrated
in Fig. 1 for the case of the OH radical. In this system,
it is seen that the OH radical is initially excited into the
v' = 1 manifold of the A22+ electronic state by ab-
sorption of laser radiation at 281.9 nm-the Qj(1)
transition. Due, however, to collisions with N2, 02, and
H 2 0, significant rotational relaxation and electronic
quenching occur, which result in both attenuated fluorescence signals and fluorescence emissions from rotational states both in the v' = 1 and v' = 0 manifolds
of the A 21+ state. Even so, the most intense fluorescence radiation is centered at 309.5 nm. 3 Thus, optical
pumping takes place at 281.9 nm, and fluorescence
sampling occurs at 309.5 nm.3 The latter approach
The authors are with Georgia Institute of Technology, School of
Geophysical Sciences, Atlanta, Georgia 30332.
Received 26 December 1979.
0003-6935/80/213597-09$00.50/0.
© 1980 Optical Society of America.
permits discrimination against Rayleigh and Raman
scattered laser radiation without a significant attenuation in the signal photon flux. It does not, however,
prevent photon noise resulting from aerosol and
chamber wall fluorescence induced by the pumping
frequency. The latter noise source, under the clean air
conditions prevalent in the natural troposphere, typically defines the dominant noise source in the conventional LIF system.
II. General Description of Photofragmentation-Laser
Induced Fluorescence (PF-LIF) Technique
A. Introduction
Although the conventional LIEF technique can be expected to provide an effective means of detecting numerous trace gases at natural tropospheric concentration levels, a great many others are not detectable by this
method due to the absence of bonding excited states that
fluoresce. In the latter case, the absence of fluorescence in the parent molecule may be overcome if the species can be made to photodecompose. Halpern et al.4
have used such an approach to detect NH3 at high sensitivity under laboratory conditions. In their system,
a high energy excimer laser, producing intense pulses
at 194 nm, is used to photolyze NH3 in a multiphoton
absorption sequence that results in the production of
electronically excited NH radicals. The resulting fluorescence from the NH* radical is then detected at 336
nm. Since this technique depends on the square of the
1 November 1980 / Vol. 19, No. 21 / APPLIED OPTICS
3597
VI *2
vw an
I
Fig. 1.
-- J
OH energy diagram showing LIF pumping and sampling
scheme.
energy density, these investigators maximize their
sensitivity by strongly focusing the 194-nm laser beam
into the center of their detection chamber. This paper
reports the details of a different photofragmentation
detection scheme, the general approach of which was
briefly described by Davis et al.3
Of particular im-
portance in the proposed new method is the fact that
much of the already developed airborne hardware used
in our previously reported LIF system 3 should also be
compatible with the PF-LIF technique. Thus, the
transition from a laboratory prototype system to an
airborne compatible unit can be expected to occur
without major engineering difficulties.
PF-LIF Approach
B.
the condition X > 3 . This preferred wavelength arrangement is in diagram form in Fig. 2. Both criteria
(4) and (5) are concerned with reducing the white fluorescence noise in the system. This noise source is
produced from atmospheric aerosols and from organic
contamination on the walls of the fluorescence chamber
upon absorbing laser radiation at XA and X2. If criteria
(4) and (5) are satisfied, all white fluorescence noise
must occur at wavelengths significantly longer than that
of the sampled signal fluorescence. Under these conditions, long wavelength rejection filters, in combination
with solar blind photomultiplier tubes (PMT), can be
used to reduce the white fluorescence noise and Rayleigh and Raman scattered radiation to the level of the
PMT dark count. This type of massive discrimination
is not possible with conventional LIF systems since
some laser induced white fluorescence noise is always
present at the central wavelength of the bandpass interference filter. Results from our laboratory, for example, show that a rejection factor of 105-106 is near
maximum for the LIF system, whereas, 1012-1014 now
appear to be practical in the case of the PF-LIF technique.
It was mentioned earlier that, in some cases, the requirement that X > 3 may not be a limiting factor in
the application of the PF-LIF technique. This is true
for a system in which the vibrationally excited photofragment formed by photolysis at A, does not undergo
rapid vibrational relaxation.5 In this case, the fluorescence pumping laser, 2 could be fired -1 sec after
the photolysis laser. Under these conditions, all the
white fluorescence noise generated by the photolysis
laser beam is decayed away before the fluorescence
AB2
V
2
V'-
I
V,*
o
.
AB
tB
fI i
In general, five criteria must be met if the PF-LIF
technique is to be successfully applied:
(1) The mo-
lecular species to be detected must have an appreciable
cross section at a wavelength Al that is accessible with
existing lasers. (2)Absorption by this parent molecule
at A must result in bond dissociation (or predissociation). (3) One of the photofragments formed in the
bond dissociation process must have a bonding excited
state that can be made to fluoresce at wavelength 3 by
pumping at a wavelength 2 again accessible with existing lasers. (4) The fluorescence inducing pumping
wavelength for the photofragment 2 should be spectrally shifted such that the condition X2 >
3
is satisfied.
This condition dictates that the photofragment be
formed in an excited vibrational state and that electronic pumping take place before significant vibrational
relaxation occurs. A fifth criterion that must be applied
to many systems (see later discussion for exceptions) is
that the photofragmentation wavelength A also satisfy
3598
APPLIED OPTICS / Vol. 19, No. 21 / 1 November 1980
X\1
V
V"-
2
>
3
* 3
2
ABE
_
V * I
V,,*
o
Fig. 2. Wavelength detection scheme of PF-LIF technique.
detecting PMT is gated on at the time of the second
laser firing (provided the laser pulse width is not much
> 10-20 nsec). 6 This delayed firing technique may
result in reduced sensitivity if, during the delay time,
the initial rotational quantum state distribution
undergoes further relaxation, thereby reducing the
maximum population available for pumping from a
single quantum state. Nevertheless, for many molecules that have significant absorption cross sections only
in the far UV, the delayed firing sequence may be essential.
Ill.
Description of PF-LIF Experimental Hardware
A laboratory PF-LIF system consists of three major
components: (1) one or two high energy pulsed dye
laser systems; (2) a fluorescence chamber with associated detection optics; and (3) sampling electronics and
data processing hardware. The use of either one or two
dye lasers to provide the photolysis and fluorescence
pumping wavelengths, X and X2, depends both on the
type of molecular species being detected as well as the
type of high energy driver laser used to excite the dye
lasers. For example, for some chemical systems either
the fundamental wavelength from the driver laser or
some harmonic of the fundamental could be used to
provide the photolysis pulse at X,. In the latter case,
only a single dye laser would be required.
The selection of the best driver laser to excite the one
or more dye lasers must be based on two criteria: (a)
the availability of short pulse widths (i.e., <15 nsec), and
(b) the availability of high energies at one or more UV
and/or visible wavelengths. The choice in this case,
therefore, is limited to either solid state Yag or excimer
gas lasers. Because of their rugged durability and very
high energies at multiple wavelengths, we believe that
a Nd:YAG system is the preferred driver laser. It
should be noted also that, even in the case where two
dye lasers are to be used to generate the X and X2 laser
pulses, a single Nd:YAG driver laser can suffice to drive
both systems if a simple optical delay line is interfaced
with the input to the 2 dye laser. For a chemical
species requiring that XI be less than X3, a second Nd:
YAG laser is needed. In the latter system, an electronic
delay is required to adjust the firing sequence of the XI
and X2 lasers.
The fluorescence chamber required for the PF-LIF
system can be of rather simple design, having the same
basic characteristics as outlined for conventional LIF
systems. The detection optics, on the other hand, must
be somewhat unique if the full potential of the PF-LIF
is to be realized. The most important features of this
proposed system are (a) the use of chemical optical filters rather than standard solid state interference
bandpass filters; (b) the use of nonimaging collection
optics rather than conventional imaging spherical
lenses; and (c) the use of solar blind PMTs. The use of
chemical filters in this system is preferred due both to
the higher transmission of a tuned multiliquid filter
system and to the relative insensitivity of this type filter
to the angle of incidence of the incoming radiation. The
latter point becomes quite important when trying to
assess the ultimate effectiveness of nonimaging collection optics. Although this type of optical system is
capable of collecting many times more light than possible with simple spherical lenses, the resulting higher
optical collection efficiency can only be fully realized
if subsequent optical filtering elements do not strongly
discriminate against off-axis rays. As noted, this is true
of simple chemical filters but not of interference
bandpass filters.
To further complement the long wavelength discrimination characteristics of the proposed chemical
optical filtering system, solar blind PMTs are also recommended. Tubes are now available with quantum
efficiencies that fall off 11 orders of magnitude in
shifting from the near UV to the visible region of the
spectrum.
The sampling electronics to support a PF-LIF system
can be rather simple in design, consisting primarily of
photon counting hardware. This system requires gating circuitry, which permits the counting electronics to
be activated in a few nanoseconds and permits variable
sampling gate widths of 20-100 nsec. Data processing
requirements for the overall system should be minimal
due to the low photon count rates expected and could
certainly be handled by a small microcomputer.
Conversion of a laboratory PF-LIF system into one
suitable for aircraft field sampling should be reasonably
straightforward if much of the basic airborne LIF
hardware 3 already in existence can be used. The PFLIF system results in an increase in weight of 35% and
an increase in electrical power requirements of -100%
over those of an airborne LIF system. A ground base
PF-LIF sampling system is obviously even more
straightforward than that needed for an aircraft platform.
IV.
PF-LIF Signal Calculations
For purposes of illustration, we consider a PF-LIF
system that detects an atmospheric species ABC by
photolyzing the molecule at a wavelength X, to form a
vibrationally excited product AB. It is assumed that
a small fraction of this photogenerated AB species is
produced in quantum state i. AB in quantum state i
is excited by a second laser, centered at wavelength X2,
resulting in the formation of an electronically excited
AB species in the j quantum state. Fluorescence from
electronically excited AB occurs at wavelength X3.
Thus, assuming that no additional AB is produced in
quantum state i from other sources, the general equation for the number of signal X3 photons detected per
laser shot can be expressed as
DX3 = (total number of X3 photons emitted P 3)
X (optical detection efficiency for X3 photons Ed)
X (electronic detection efficiency Ee).
In abbreviated form
DX3 = PX3 X Ed X Ee.
(1)
The simplest term in Eq. (1) Ee represents the fraction of the total PMT signal detected by the counting
electronics:
1 November 1980 / Vol. 19, No. 21 / APPLIED OPTICS
3599
E = (PMT signal pulses counted Pd)
(PMT signal pulses emitted P.)
(2)
In most cases, it is expected that the value for this term
is unity. 7
The second term in Eq. (1), the optical detection efficiency, may be defined as
(fraction of total fluorescence at samplin
wavelength X3,y
m
)
E
X (collection optics efficiency factor at X3 , YX3)
X3, ZXA)
X (quantum efficiency of PMT, X,
X (filter transmission factor at
or
Ed = YX3 X YX3 X Z3 X OX
3
(3)
The first term in Eq. (1) is more complex than those
given above and is presented here in several steps. The
most general expression for PX3 is given here in the form
of Eq. (4):
PX3 = (total number of X2 photons absorbed Nx 2)
X (fluorescence efficiency Ef).
(4)
The number of X2 photons absorbed by photofrag-
The exponential term in Eq. (9) accounts for saturation
effects that are sometimes important in the photolysis
step. The equation presented here is based on an absorbing sphere model, which is valid for low concentrations of absorbers. In this equation, Px1, ¢X1, and
ax identify the
laser photon flux, the absorption
cross section of ABC molecules at X, and the X laser
beam cross-sectional area, respectively. The Q. term
in Eq. (9) is the primary quantum yield for production
of AB by photolysis of ABC, whereas, fi represents that
fraction of the newly formed AB species that is in the
desired quantum state i. The variable F is a correction
term designed to take into consideration any change in
the population of the ith quantum state that might
occur in the time interval between the firing of the X
and 2 lasers. This term, therefore, corrects for the
effects of vibrational and rotational relaxation. For
short (<20-nsec) time delays, the value of this term
should be near unity. 8
The second term in Eq. (5), the fluorescence pumping
efficiency Ex2, must take into account both the fraction
of molecules absorbing photons within the X2 beam and
the imperfect overlap of the
and A2 laser beams.
Applying the Beer-Lambert law, we find
ment AB molecules within the sampling volume may
be expressed as
NX =(total number of AB molecules in quantum
(state i within sampling region C
{fraction of molecules in quantum
state i absorbing photons Ex 2
J
I
(5)
The total number of AB molecules in quantum state
i within the sampling region can be related to the photolysis pumping scheme and the number of precursor
molecules by Eq. (6):
C
(number of ABC molecules)
in sampling region Cr
fraction of ABC molecules
X
undergoing photolysis and producing
photofragments in quantum state i
[ABC]:
aX2
(10)
-
ax,
equation assumes that the A2 beam is completely over-
lapped by the X beam and that the fraction of photofragments absorbing photons is small enough that a
single term Taylor series approximation to the BeerLambert law yields acceptable results.
Returning finally to the second term in Eq. (4), Ef, it
is recognized that this quantity can be expanded according to Eq. (11), i.e.,
fraction of excited
molecules producing}
fluorescence
kf
[kf +kd + kq [MI]
(11)
where kf is the reciprocal of the natural radiative lifetime r of AB in the ith excited state, kd is the first-order
electronic quenching rate constant, and [M] is the cor-
centration of the quenching species.
X concentration of the precursor)
species [ABC]
P
Based on detailed Eqs. (5)-(11), Eq. (4) can now be
rewritten in the following form:
The volume V is given by
V = a, X 1,
(8)
where I is the laser path length, and ax, is the
beam cross section.
laser
The photolysis efficiency Ex, from Eq. (6) is defined
as
3600
X
rate constant for dissociation, kq is the bimolecular
C, = (volume of sampling region V)
- exp - PA1ux\) x Q x fx
aX2
where PX2 is the A2 laser photon flux, AX
2 is the absorption cross section for the photofragment at X2, ax, and
ax2 are the cross-sectional areas of the photolysis and
the fluorescence pumping beams. This form of the
Ef =
The number of ABC molecules within the sampling
1
PX2 X OA2
(6)
region C is given by the product of the sampling volume V and the concentration of the precursor species
Ex =
EX 2 =
F.
(9)
APPLIED OPTICS / Vol. 19, No. 21 / 1 November 1980
P, 3 = Ex, X Ex2 X El X V X [ABC].
(12)
And, upon substituting Eq. (12) into Eq. (1), the final
signal expression becomes
Dx3 = Ex, X Ex 2 X Ef X Ed X E X V X [ABC].
(13)
Equation (13) is a very useful form of the signal equation since each term within it has an easily identified
physical significance, for example,
P
photolysis\
Ex, =
efficiency
=
-
exp
optical pumping
EX2
=
E
2
) X fi X F;
X
X
2l
=
efficiency
at 2
efficiency
PX
-
ax2
ax,
[kf I
I-fluorescence
I
[kf + kd + kq[M]]
optical
Ed = detection = 'YX
2 X Yxs X Zx2 X x,;
efficiency/
(electronic
E=
detection
efficiency
=
Pd
S
volume of
V
sampling
The second category of noise involves sources that are
directly related to the transmission of one or both laser
beams through the fluorescence sampling chamber.
For purposes of clarity, we have further divided this
second category into two subcategories: (1) photon
noise occurring at wavelengths 2X2 ; and (2) photon
noise occurring at wavelengths <X2 - Sources in the first
subcategory include white fluorescence noise from gas
phase molecules, aerosols, and wall contaminants;
Stokes Raman scatter from gas phase molecules; and
Rayleigh scatter from both gas phase molecules and
aerosols. For the proposed PF-LIF system, the latter
noise source, in principle, could be reduced to levels
equal to or less than that of the PMT dark count via a
combination of long wavelength chemical filters and
solar blind PM tubes.
The second subcategory of laser induced noise (i.e.,
sources that generate noise wavelengths <X2 ) is obviously more difficult to eliminate. At this time, two
possibilities have presented themselves. The first could
involve multiphoton short wavelength fluorescence.
This type of fluorescence could result from any one of
five basic processes, i.e.,
=ax, X 1.
region
From an examination of Eq. (13) in terms of the detailed expressions for Ex,, EX2, Ef, Ed, Ee, and V, two
important characteristics can be defined regarding the
relationship between signal photons detected and the
concentration of ABC. (1) To a first approximation,
the signal depends upon the average value of the product of the laser photon fluxes. Since the average value
of the product of these two quantities is generally not
equal to the product of the individual average values,
that is,
(PxPx2 ) D (PX,) X (PX
2 ),
either a measurement of both laser energies for each
laser firing is required or a reliable description of the
energy distribution statistics for each laser is needed.
(2) In the case of a completely overlapped X2 beam, the
signal is seen to be independent of the X2 beam cross
section.
V. Evaluation of Photon Noise Sources and
Chemical Interferences
In a PF-LIF system, noise sources may be divided
into two general categories. First, there are those
sources that are independent of the transmission of either laser beam through the fluorescence sampling
chamber. Sources in this category include light leaks
in the PMT housing, the dark count of the PMT, and
natural background radiation entering the fluorescence
chamber through the laser entrance port. With the use
of short time gates and careful mechanical design, we
now estimate that the collective count rate from these
sources is negligibly small (in the 10-6 -10- 7 -counts/
laser shot range).
hv2
(a)
AB 2 + hv 2 -[AB
2] -- )AB2
AB* - AB 2 + hv3
(b) AC 2 + hv2
AC*
-A AC*2
hv2
(V3 > V2),
AC-
AC 2 + hv 4
(V4 > V2),
M
(c)
AC 2 + hv 2 - ACACI
(V5 > V2),
AC + hv2 - ACI* - AC 2 + hv 5
(d)
AC + hv2 - AC2 - AC + hv6
AC* - AC 2 + hv 7
ACI + hv2
(e) AD 2 +hv 2 - AD2
hv'2
(v6 < V2)
(7
>
V2),
AD* +D
AD* - AD + hvs
(v8 > V2)
In the above schemes V2 is the pumping laser frequency,
[AB 2 ] represents a virtual excited state, AB* is electronically excited AB 2 , AC** is a bonding upper electronic excited state of AC 2 , AC is vibrationally excited
ground electronic state AC 2 , AC* is a highly vibrationally excited state of the first excited electronic state
of AC2 .
The above two-photon noise sources are by far the
most difficult to evaluate due to a lack of quantitative
data on photochemical quantum yields, quenching cross
sections, absorption cross sections, Franck-Condon
factors, and atmospheric concentration levels. However, for purposes of obtaining order or magnitude estimates of this noise source, we examined here the minor
atmospheric constituents N 2 0, C0 2, H2 0, CO, and CH4
plus the fluorescing and nonfluorescing simple polyatomic trace gases SO2 , CS2 , CH 2 0, H2 02 , CH 3 00H,
NH3 , HNO3 , H2 S, and COS. Finally, several representative complex species were investigated; these include acetaldehyde, butylaldehyde, acetone, benzene,
1 November 1980 / Vol. 19, No. 21 / APPLIED OPTICS
3601
napthalene, and anthracene. Our general findings are
that process (a) was always found to be unimportant as
a noise source because of the low cross-section values
inherently involved in two-photon absorption processes.
Calculated noise signals from process (b) are significantly greater than those derived from (a) but still are
typically lower than the PMT dark count. The low
efficiency of process (b) reflects the high probability for
bond dissociation when molecules are energized by two
photons. Process (e) has been estimated to be more
important than (b) but usually only for that case where
V8 < 2 . A study of the latter type of system was reported by Halpern et al.4 in which excited NH was
formed from the photolysis of NH3 (see Sec. II. for details). Potentially, the largest source of two-photon
noise comes in the form of processes (c) and (d). For
these processes, the most important molecules are believed to be the strong UV absorbing species napthalene
and anthracene. If present in the gas phase at the pptv
concentration level, it is estimated that photon noise
levels as high as 10-6 /laser shot result. Although we
presently believe that this is a worst case estimate, the
possible uncertainties in the values used for the internal
conversion quantum yields and for quenching seem to
justify a very careful examination of these processes
under controlled laboratory conditions.
The second low wavelength noise source (i.e., Xnoise
< 2 ) is anti-Stokes Raman scattering. The molecules
N2 , 02, and CO 2 are, in general, the dominant antiStokes Raman scattering sources in atmospheric sampling due primarily to their high ambient concentrations. Calculations indicate that the minimization of
this noise source requires the selection of a laser
pumping scheme that maximizes the separation between the pumping and sampling wavelengths, for
purposes of discrimination, but does not result in major
losses in signal counts due to poor Franck-Condon
factors. In general, if the separation between 2 and X3
is at least 20 nm, chemical filters can be designed that
strongly discriminate against first-order anti-Stokes
lines and, to some degree, second-order anti-Stokes
noise. Even so, present calculations indicate that
anti-Stokes Raman scattering probably define the lower
limit on detection sensitivity for a PF-LIF system.
In addition to the real noise sources outlined, one
must also consider interference signal photons derived
from chemical species other than the one specifically
being monitored, in our example, ABC. If, for instance,
the photofragment AB also occurs naturally in the atmosphere, a potential problem could develop if the
Boltzmann distribution of this naturally occurring
species contained a significant number of molecules in
the same quantum state i used for fluorescence pumping. This interference, however, can be eliminated by
the proper selection of the AB quantum state i. If the
i state, for example, consists of AB molecules energized
by two or more vibrational quanta, the naturally occurring population at this energy is expected to be
negligible.
The second type of chemical interference that can
3602
APPLIED OPTICS / Vol. 19, No. 21 / 1 November 1980
occur in the PF-LIF system is that involving the production of the photofragment AB molecule from the
photolysis of an atmospheric trace gas other than ABC.
In this case, some fraction of the AB formed could be
produced in quantum state i, and these species would
be indistinguishable from those produced from ABC.
Even so, the selection of XI and X2 provides a great deal
of flexibility in eliminating interferences of this type.
For example, by varying the photolysis wavelength, the
absorption cross section for molecule ABC, relative to
that for one or more possible interfering species, could
very likely be changed by factors ranging from 2 to 10.
Thus, the magnitude of the interference signal could be
quantitatively defined and then eliminated, if necessary,
by selecting a new photolysis wavelength. Equally
important is the potential selectivity afforded by
varying X2, the fluorescence pumping wavelength.
Since it is highly probable that each atmospheric trace
gas photodissociated at wavelength XI produces fragment molecules having a unique distribution of rotational and vibrational quantum states, varying the
pumping wavelength X2 should provide a clear identification of the molecular species being sampled.' 0 The
latter type of selectivity, however, necessitates a rather
complete advance knowledge of the rotational distributions within one or more vibrational levels of any
molecular species believed to be a potential problem.
In most cases, it is expected that serious chemical interferences could be avoided by simply comparing the
relative concentration levels of the species involved, and
the relative absorption cross sections, and the energetics
for bond rupture and photofragment excitation. This
simple approach was used in formulating the pumping
schemes outlined for several molecules in the next section.
VI.
PF-LIF Sampling Schemes
Within the NO. family there are at least three species
that appear amenable to detection via the PF-LI F approach. When photolyzed in the visible and/or near
UV, the gases NO2 , NO3 , and HNO2 all produce photofragments capable of fluorescence. Here we explore
possible PF-LIF photolysis and pumping wavelength
schemes for each of these gases.
A.
NO 2
(a) NO2 + hv(X = 300 nm) - NO(X 211,V"
(b)
= 2)
+ 0(3P),
NO(v" = 2) + h 2 (X2 = 248 nm) - NO(A 2F+,v' = 0),
(c) NO(A 2 j+,V' = 0) - NO(X 211,v" = 0) + hv3 (X3 = 226 nm).
The selection of 300 nm for X, in process (a) involves
a compromise between the NO2 absorption cross section," the achievable photon flux, and the fraction of
the NO photofragment population that can be produced
in the v" = 2 vibrational manifold. The wavelength
assignment of 300 nm, therefore, represents a near
maximum in the product of the achievable photon flux
from a frequency doubled dye laser and the absorption
cross section for NO2 in a wavelength region believed
optimal for populating the v" = 2 state of NO.
Wavelength selections for X2 and X3 are based on
consideration of the values for 0 X2, P 2 and the
Franck-Condon factors for the several possible electronic transitions for NO. Those selected here tend to
maximize the value of Dx3 without resulting in proportionate increases in the anti-Stokes Raman noise
level. As seen in scheme (a)-(c), the assignment of
248-nm radiation for X2 and 226 nm for the sampling
wavelength also makes possible a 22-nm separation
between X2 and X3. Thus, the use of the v" = 2 vibrational manifold as the origin for process (b) allows for
large scale discrimination against the first anti-Stokes
Raman lines from N2 , 02, and CO2 and even limited
discrimination against second-order anti-Stokes radiation from 02 and CO2. The second-order lines from
N2 are sufficiently weak as to be of negligible importance as a noise source.
B. NO3
The wavelength selection of 248 and 226 nm for X2
and X3 in the NO2 system can also be used in the detection of NO from NO3 . In the latter system, however,
due to the very low natural concentration levels of NO3 ,
a photolysis wavelength X,must be selected that energetically can produce v" = 2 NO photofragments from
NO3 but not from the far more populous NO, species
such as NO2 or HNO3 . Based on the known photochemistries for these three species, the desired energy
configuration can be achieved by selecting photolysis
wavelengths that are 580 nm. Neither NO2 nor
HNO 3 undergoes bond rupture at these wavelengths.
For NO 3 , the process
2
(d) NO 3 + hv(Xi = 589 nm) - NO(X 11,V" = 2) + 02 (0A)
should occur with high probability. The exact assignment of 589 nm as the photolysis wavelength was determined from available information on the absorption
cross section for NO3 , given by Johnston and Graham,12
and from the quantum yield values recommended by
Magnotta.13
C.
HNO 2
The detection scheme for HNO2 , like that for NO 3 ,
can involve X2 and X3 assignments of 248 and 226 nm,
respectively. Once again, however, NO in the v" = 2
level needs to be formed from the photolysis of HNO 2
at a wavelength that energetically could not produce
this same quantum state of NO from NO2 or HNO 3 . In
this case, the selection of the third harmonic Nd:YAG
(i.e., 355 nm) appears to be a unique assignment since
(1) HNO 2 has an absorption maximum near this wavelength; (2) very high photon fluxes can be generated at
355 nm; (3) the HNO3 absorption cross section is -3
orders of magnitude lower than that for HNO 2 , and
available photochemical evidence also suggests that NO
is not a primary product in the photolysis of HNO3 at
long wavelengths; (4) absorption by NO2 can only produce NO in the v" = 2 level provided all excess energy
were to go into vibrational excitation, a highly improbable event; and (5) the absorption cross section for
NO 3 is very low at 355 nm, and photolysis at that
wavelength results in a negligibly small quantum yield
for production of NO. The proposed method for detecting HNO 2 may therefore be summarized as
(e)
HNO 2 + hv(Xi = 355 nm)
2
2
NO(X 11,v" = 2) + OH(X 11)
followed by processes (b) and (c) given earlier. In the
HNO 2 system, the possibility also exists that OH could
be used as the monitored photofragment; however, the
data reported by Cox et al.' 4"15 suggest that in the
photolysis of HNO2 at 355 nm, very little excess energy
is partitioned into internal energy involving the OH
species. If true, this results in a decrease in the separation between X2 and X3 and thus significantly reduces
the sensitivity of the PF-LIF technique.
VIl.
Other Species
At the present time, the number of trace gases that
potentially could be detected via the PF-LIF technique
appears to be quite large. Among the possible candidates are H 2 02 , HNO3 , CH3 00H, CH3 I, CH 3 Br,
CH 30NO, CS2 , CH 3 SH, NH 3 , and INO3 . Detectable
photofragments consist of OH, I, Br, NO, CS, SH, NH,
or NH2 and NO or 10. Future advances in the spectroscopy and photochemistry of each of the above parent molecular species should thus provide a basis for
evaluating PF-LIF detection schemes.
Vil.
Sensitivity Calculations for Three NO, Gases
Calculations are presented that estimate the detection sensitivity of the PF-LIF system for NO 2, NO 3, and
HNO2 . The choice of these three species was based
both on there being available reasonable estimates of
the absorption cross sections and the fact that, for all
three systems, NO could be selected as the photofragment from which fluorescence is monitored. In these
calculations, we also made the simplifying assumption
that in each case quantum state i is the NO state from
which fluorescence pumping occurs. This simplification allowed us to establish several common parameters
for each system investigated. For example, since the
terms Ef, Ed, and Ee do not depend upon the characteristics of the X, or X2 laser beams, their values can be
taken to be the same for each NO. system. The product E 2 X V is also independent of the X,laser; however,
to establish specific values of EX2 and V, we made the
further assumption that ax is the same for all three
detection systems. Estimated values for these common
terms are in Table I. Ex1 has not been included in
Table I since this quantity must be estimated independently for each NO. species being considered. In
Table II we summarized the estimated values of Ex1 for
each NO. system, assuming negligible vibrational and
rotational relaxation. Finally, Tables I and II were
combined to yield mathematical expressions that give
the detected signal as a function of the ambient concentration (in molecules/cc) of the parent NO. species.
The appropriate equation for each of these systems can
be expressed as
1 November 1980 / Vol. 19, No. 21 / APPLIED OPTICS
3603
Table 1. Estimation of E,,, E,, Ed, E., and V for Detection of NO2, NO3,
and HNO2
Term
Variables
Ref.
E,\2 = 3.4 X 10-2
Px2 = 2.1 X 1015 photons
ax, = 8 10-1 cm 2
a,, = 0.5 cm2
Assumed
18
Assumed
Ef = 7.1 x 10-3
1
kf = 6.3 X 106 seckd = 0
k = 1.7 x 1010 cm3 /sec
[Ml = 5.2 X 1018 cm-3
19
Ed = 1.4 X 10-4
20
21
22
7YX
3 = 0.16
YX3 = 0.09
ZX3 = 0.05
OX3 =
23
Assumed
Assumed
Assumed
0.20
Ee = 1.0
V = 7.5
Assumed
10-1 cm 3
Table II.
x,
Species (X1016 )
1.1
8.9
7.4
ax = 0.5 cm 2
I = 1.5 cm
Assumed
Assumed
Estimation of ER for Detection of Three NO, Species
Px, I
NO 2
HNO 2
NO 3
NO 3 could be detected at their natural levels in the
unpolluted troposphere with reasonably good time
resolution. On the other hand, the natural concentration level of HNO 2 appears sufficiently low that detection of this species is doubtful. The detection of HNO2
at the few pptv level, however, could prove useful for
other applications. For instance, the free radical
species HO 2 is estimated present in the atmosphere at
concentration levels of 2-40 pptv. This important atmospheric free radical is also known to react very rapidly with NO according to the reaction HO 2 + NO NO 2 + OH. Thus, if high concentrations of NO were
injected into a sampled airstream, the HO2 is first
converted into OH and NO2 followed by the production
fi
(cm2 )1 1
(X10- 2 )
1.3 X 10-19
1.2 x 10-19
5.5 X 10-'8
1.5
1
2
Q
F
1.012
0.914
EXI
1.0
1.0
0.3513 1.0
4.3 X 10-5
1.9 X 10-4
3.9 X 10-3
M
of HNO2 via the very fast reaction OH + NO HNO2 .
Since HO 2 is present in the atmosphere at concentrations >1 order of magnitude higher than natural OH,
the HNO2 formed in the above reaction presents itself
as a unique diagnostic molecule, which potentially could
be used to determine the atmospheric concentration
level of HO2 . Even the NO2 formed in reaction NO +
HO2 - NO2 + OH could be used as an effective monitor
of HO 2 provided the natural tropospheric NO2 level was
not much different than that of HO2 . Potential complications resulting from other peroxy radicals (i.e.,
CH3 0 2 ) could be resolved by measuring the concentrations of both titration products NO2 and HNO2 .
IX.
D,
=
1.1 X 10-
12
3
cm X [NO 2 ],
Dx3 = 2.5 X 10-1 2cm3 X [HNO2 ],
Dx3
=
9.9
10-
11
cm
3
X [NO 3 ].
In Table III the signal strength expressions given
above were combined with anti-Stokes Raman noise
estimates to permit an evaluation of the SNR for each
NO, species. Several different ambient concentration
levels and integration times are included. These results
suggest that for integration times of -5 min., 3 pptv (at
sea level) of NO2 could be detected with a SNR of 2:1.
For NO3 and HNO2 , the concentrations required to
produce SNRs of 2:1 for the same integration time
correspond to 0.3 and 2 pptv, respectively. These estimated detection sensitivities indicate that NO2 and
Table
Species
NO 2
HNO2
NO3
3604
.
Summary and Conclusions
In this paper we outlined a new method for detecting
nonfluorescing atmospheric trace gases. This technique appears both highly sensitive and specific. The
general equations for calculating the signal strength
were given. Possible sources of noise as well as chemical
interferences were discussed. Based on available information, the dominant noise source in the PF-LIF
system was identified as anti-Stokes Raman scatter
involving the atmospheric species N 2, 02, and CO 2 . It
is also noted, however, that multiphoton fluorescence
noise might later prove significant in some systems.
Extensive experimental testing is required to resolve
this question.
Detection schemes for employing the PF-LIF system
in the monitoring of several NO. species were presented. Using the signal equations developed here,
PF-LIF Signals for Detection of Three NO, Species
Number of
laser shots
(min at
20 Hz)
Estimated
signal
counts
Estimated 2 4
noise
counts
Signal
real
noise
Signal 2 5
statistical
noise
5 X 108
2.5 X 108
7.5 X 107
6000 (5 min)
12000 (10 min)
24000 (20 min)
20
20
12
0.5
1
2
40:1
20:1
6:1
4.5:1
4.5:1
3.5:1
5 X 108
1.25 X 108
2.5 X 107
6000 (5 min)
12000 (10 min)
12000 (10 min)
46
23
30
0.25
0.5
1
184:1
46:1
30:1
6.8:1
4.8:1
5.5:1
2.5 X 107
7.5 X 106
2.5 X 106
12000 (10 min)
24000 (20 min)
24000 (20 min)
30
20
7
1
2
2
30:1
10:1
3.5:1
5.5:1
4.5:1
2.6:1
Concentration
APPLIED OPTICS / Vol. 19, No. 21 / 1 November 1980
along with estimated noise levels, we carried out detailed calculations to determine the detection sensitivity
of the PF-LIF technique for the gases NO2 , NO 3 , and
HNO 2 . While the detection sensitivity for HNO2 (5 X
107 molecules/cc for 5 min of integration) does not appear sufficient to measure this species at its natural
levels in the unpolluted troposphere, the sensitivity
limits on both NO 2 and NO 3 now appear more than
adequate. The calculations presented suggest that for
a 5-min integration time period the detection limits for
NO2 and NO3 at sea level should be 3 and 0.3 pptv, respectively.
In addition to in situ atmospheric concentration
measurements of trace gases, the PF-LIF technique
may be amenable to other applications. For example,
studies of molecular photodynamics, especially in the
study of energy partitioning in photofragments, may be
advanced with the application of this methodology.
The authors would like to acknowledge NASA grant
NAGL-50 which provided funding for this work.
Appendix: Symbols
Ex,
cross-sectional area of X1 beam;
cross-sectional area of X2 beam;
concentration of parent species;
number of AB molecules in quantum state
i within sampling region;
number of ABC molecules in sampling
region;
number of signal counts/laser shot;
optical detection efficiency for X3 photons;
electronic detection efficiency;
fluorescence efficiency;
photolysis efficiency;
EX 2
fluorescence pumping efficiency;
fi
fraction of photofragment AB in desired
quantum state i;
vibrational and rotational relaxation
correction;
Franck-Condon factor of desired
fluorescence transition;
first-order rate constant for dissociation of
AB in the jth excited state;
bimolecular rate constant for electronic
quenching of AB in the jth excited state by
molecule M;
reciprocal of the natural radiative lifetime
of AB in the jth excited state;
laser path length through sampling region;
photolysis wavelength;
fluorescence pumping wavelength;
fluorescence wavelength;
concentration of quenching species;
number of photons absorbed by AB
molecules within the sampling region;
number of signal pulses counted by
detection electronics;
X1 photon flux;
X2 photon flux;
number of fluorescence photons emitted;
axI
ax2
[ABC]
Ci
C,
Dx,
Ed
Ee
Ef
F
lYX 3
kd
kq
kfi
1
Al
X2
X3
[M]
NX 2
Pd
Px 1
PX2
P, 3
P.,
Q.,
ax
1
AX
2
V
YX3
ZX3
number of signal pulses outputted from
photomultiplier tube;
quantum efficiency of the photomultiplier
tube at X3;
primary quantum yield for production of
AB by photolysis of ABC at X,;
absorption cross section for ABC at Xi;
absorption cross section for AB at X2;
volume of sampling region;
optical collection efficiency for X3 photons;
filter transmission at X3 .
References
1. K. Sukurai and H. P. Broida, J. Chem. Phys. 50, 2404 (1969).
2. See, for example, W. M. Jackson, J. Chem. Phys. 50,960 (1973);
J. A. Silver, W. L. Dimpfl, J. H. Brophy, and J. L. Kinsey, J. Chem.
Phys. 65, 1811 (1976); J. A. Gelbwachs, M. Birnbaum, A. W.
Tucker, and C. L. Fincher, Opto-Electronics 4, 155 (1972).
3. D. D. Davis, W. S. Heaps, D. Philen, M. Rodgers, T. McGee, A.
Nelson, and A. J. Moriarty, Rev. Sci. Instrum. 50, No. 12, 70
(1979).
4. J. B. Halpern, W. M. Jackson, and V. McCrary, Appl. Opt. 18,590
(1979).
5. That is, having Ile time constants for vibrational relaxation >1
,usec at atmospheric pressure.
6. This also assumes that the RC time constant of the signal cable
and related input-output detection electronics is <20 nsec.
7. This will be true if two conditions are satisfied: (1) the signal lost
in output signal gating is negligible; (2) impedance mismatches
that result in reflections in the signal cable terminations are small
so that, after two reflections, pulses on the signal cable are undetectable.
8. Most diatomics have le relaxation times from 5 to 20 times longer
than this time delay.
9. Interfering AB molecules might also be formed by secondary
photochemistry involving the XAand/or X2 laser beams.
10. This technique of using vibrational and rotational population
distributions to "fingerprint" molecules may have widespread
use as an analytical tool for identification of species.
11. E. R. Reiter, "The Natural Stratosphere of 1974," CIAP Monograph I, DOT-TST-75-51 (1975).
12. H. S. Johnston and R. A. Graham, J. Phys. Chem. 77, 62
(1973).
13. F. Magnotta, unpublished results (1979).
14. R. A. Cox, J. Photochem. 3, 291 (1975).
15. R. A. Cox, J. Photochem. 3, 1975 (1975).
16. In the (2,2) transition, k' levels <15 want to be excited to prevent
extensive predissociation of the OH.
17. A. B. Callear, Proc. R. Soc. London Ser. A: 276, 401 (1963).
18. Extrapolated from the work of T. Tajime, T. Saheki, and K. Ito,
Appl. Opt. 17, 1290 (1978).
19. Corresponding to an assumed natural radiative lifetime of 160
nsec.
20. G. Herzberg, Spectra of Diatomic Molecules (Van Nostrand
Reinhold, New York, 1950).
21. L. A. Melton and W. Klemperer, Planet. Space Sci. 20, 157
(1972).
22. U.S. Standard Atmosphere (U.S. GPO, Washington, D.C.,
1966).
23. R. J. Spindler, L. Isaacson, and J. Wentink, J. Quant. Spectrosc.
Radiat. Transfer 10, 621 (1970).
24. Estimated to be due principally to residual anti-Stokes Raman
scattering from the X2 laser beam.
25. If the signal is assumed to follow a Poisson distribution, the
standard deviation is given by a = aN, where N represents the
total number of counts. This column thus gives the ratio of the
signal to the statistical counting uncertainty.
3605
1 November 1980 / Vol. 19, No. 21 / APPLIED OPTICS