Download H3O Overtone Spectroscopy

WDS'06 Proceedings of Contributed Papers, Part II, 96–101, 2006.
ISBN 80-86732-85-1 © MATFYZPRESS
H3 O+ Overtone Spectroscopy
P. Hlavenka, I. Korolov, R. Plašil, J. Varju, T. Kotrı́k, and J. Glosı́k
Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.
O. Votava
J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of Czech Republic,
Prague, Czech Republic.
Abstract. In this report we present preliminary results in search for the H3 O+
spectra in the first OH stretching overtone region. At least 30 absorption lines of
H3 O+ were found in near infrared region 6900 to 6960 cm−1 using a very sensitive
time resolved continuous wave cavity ringdown spectrometer (cw-CRDS) with
concentration modulation. H3 O+ ions were produced in pulsed microwave discharge
in He/H2 O mixture. A technique based on time resolved absorption has been used
to distinguish the absorption lines of short living ions from longer living radicals
and water vapors.
Introduction
Hydronium ion, H3 O+ , is one of the key ionic species in the both gas and condensed phases.
Besides its importance in both chemistry and astrophysics, H3 O+ also serves as an important
model system for benchmark theoretical predictions. Due to its small size high level theoretical
treatment is possible for this molecule. H3 O+ is isoelectronic with ammonia, NH3 , and shares
number of important characteristics with this molecule. Thus the theoretical tools developed
for characterization of ammonia can be put to an independent test when applied to H3 O+ .
Equilibrium geometry of both NH3 and H3 O+ molecules is a shallow symmetrical pyramid. Consequently, the bending potential in the dihedral angle exhibits symmetric double well
structure with a barrier at planar configuration. Due to tunneling through this barrier all the
vibrational states in this inversion-vibration are split into doublets. The magnitude of this
splitting is a sensitive measure of the inversion barrier height, and thus provides a stringent test
on the theoretical predictions. Indeed, despite the general topological similarity between NH3
and H3 O+ , the observed tunneling splitting magnitude is dramatically different: 0.793 cm−1
and 55.35 cm−1 for NH3 and H3 O+ respectively.
High resolution spectroscopy provides some of the most accurate experimental data, which
can be used to test the theoretical predictions. Vibrational spectroscopy of the H3 O+ ion has
therefore attracted substantial attention over past two decades. The first high resolution spectrum of H3 O+ was reported by Saykally and coworkers in 1983 for the asymmetric stretch, ν3 ,
fundamental band in the mid IR spectral region [Begemann et al., 1983] and this was followed
by more detailed studies of the same band [Uy et al., 1997; Bunker et al., 1984; Spirko et
al.,1989; Begemann et al.,1985]. The much weaker symmetric stretch, ν1 has also been detected
and analyzed [Tang et al., 1999] in the same spectral range. The doubly degenerate asymmetric deformation mode ν4 was first detected by Gruebele in 1987 [Gruebele et al., 1987]. The
inversion-tunneling mode has also received considerable attention. The absorption spectra of 0,
1± , and 2± were measured by several groups [Davies et al., 1986; Haese et al., 1984; Liu et al.,
1985].
Accurate calculation of the large inversion splitting in H3 O+ has indeed proved a challenge
for theorists. Reduced-dimensionality calculations were capable of semi-quantitative description
of the vibrational-inversion dynamics [Spirko et al., 1989; Bunker et al., 1984] yet they lack the
information on couplings between the different vibrational modes that lead to effective change
in the barrier height as a function of vibrational excitation. Thus only the most recent full
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HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY
dimensional ab initio calculations of the potential energy surface (PES), coupled with variational evaluation of the vibrational states yielded satisfactory agreement with experiment in
the fundamental and several low lying combination bands [Rajamaki et al., 2003; Huang et al.,
2002].
While the agreement between theories based on the full dimensional ab-initio PES and the
available data is very good, there is presently a clear lack of experimental data that would test
the PES at higher energies that are crucial for detailed understanding the chemical properties
of the H3 O+ ion. Molecular overtone spectroscopy is inherently a probe into those parts of
molecular potential energy surfaces and thus is capable providing this critical information on
the chemically relevant energy levels. Unlike the fundamental bands, the overtone transitions are
strictly forbidden in the harmonic limit of the potential, and therefore their non-zero oscillator
strengths are derived from the anharmonic effects. Moreover due to the higher energy of the
excitation photons a wider range of the PES is sampled. In the case of high frequency stretching
modes in hydrogen contained bonds, such as O-H, C-H, and N-H, chemically significant levels of
excitation on the order of 1 eV can be achieved with only few quanta of vibrational excitation.
From the experimental point of view however, the orders of magnitude lower oscillator strengths
in the overtone bands as compared to the fundamental bands make it often challenging to detect
the weak molecular absorptions. This is especially the case for short-lived transient species such
as molecular radicals or ions, because concentrations at which those species can be prepared in
the laboratory are typically orders of magnitude lower than for stable molecules. As a result,
the overtone spectroscopy is not well characterized for many important species.
In this report we present preliminary results in search of the H3 O+ spectra in the first OH
stretching overtone region. The experiments were made possible by use of very high sensitivity
spectroscopic techniques based on time resolved cw cavity ringdown scheme coupled with AC
microwave discharge absorption cell. Based on the time evolution of the absorption signals
it is possible to distinguish the weak ionic absorptions from those of much stronger signals
originating from the stable or metastable species in the gas mixture.
Overtone band position predictions
Due to its near planar geometry and C3 v molecular symmetry H3 O+ is an oblate symmetric
top molecule. Because of the complexity of the symmetric top spectra that is further complicated by the presence of l-type doubling in the degenerate ν3 vibration and by the tunneling
splitting of the vibrational bands. High number of lines of comparable intensity is observed
over a wide range of spectra. The initial assignment of the spectra is further complicated by
limited tunability and incomplete coverage of the spectral range scanned with the diode laser
spectrometer. Consequently it is crucial to use a systematic, theory assisted approach to the
initial assignment of the spectra.
To guide the initial experimental search of the overtone spectra, estimates of the band
positions are calculated based on fundamental band frequencies and empirical estimates for
anharmonicity constants. The Morse-like anharmonic energy levels can be written as
En = ωe (0.5 + n) − ωe xe (0.5 + n)2 .
(1)
Thus the absorption energy for the first overtone transition (ν = 2 ← 0) is given by
∆E2←0 = 2(∆E1←0 − ωe xe ).
(2)
Where ∆E1←0 is the fundamental band origin. Estimate of the anharmonicity factor ωe xe
is made by analogy with NH3 overtone bands, scaling the factor by the ratio of fundamental
vibrational energies of NH3 and H3 O+ respectively. As those fundamental energies are rather
close for those two species (3443 cm−1 and 3555 cm−1 for NH3 and H3 O+ respectively [Rajamaki
et al., 2003; Lundsberg-Nielsen et al., 1993]), we assume that the error introduced by this
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HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY
Figure 1. Schematic diagram of the apparatus. The pulsed microwave discharge takes place
in silicate glass tube. For CRDS two highly reflective mirrors are mounted on tilt stages inside
the vacuum.
estimate will be only on the order of few cm−1 , and therefore will not affect the spectroscopic
search. The anharmonicity estimated by this procedure for the ν3 asymmetric stretch is 48 cm−1
and therefore for the fundamental origin energy of 3555 cm−1 , the expected 2ν3 overtone origin
should be near 7014 cm−1 . It is rather likely that the (ν1 + ν3 ) combination band will lie in the
same spectral range, and assuming the same effective anharmonicity we can estimate its origin
near 6927 cm−1 . Based on those estimates the initial spectroscopic search was performed in the
6900 cm−1 to 6960 cm−1 spectral range.
Experiment
The H3 O+ ions are formed in microwave discharge in He/H2 O mixture. At pressure of
5 mbar (500 Pa) the flow of gases was: He 400 sccm (standard cubic centimeter per minute)
and H2 O 0.5 sccm. The water vapors were evaporated from a free water level in a small chamber
at room temperature; we used doubly distilled deionized water. The water was initially frozen
to −30
and chamber was evacuated to remove the air. The plasma is ignited by pulses of
microwave (4 ms discharge, 100 Hz repetition, 60 - 120 W, 5 cm plasma column) in a silicate
glass tube ( 2.3 cm inner diameter) passing through a microwave cavity, see Fig. 1. Aside from
H3 O+ the OH radical and of course H2 O absorption lines have been observed.
The recently build cw cavity ringdown spectrometer (cw-CRDS) had been used in past for
the time resolved absorption measurements of H+
3 ion and its deuterated analogues using weak
second overtone transitions [Macko et al., 2004; Hlavenka et al., 2006a; Hlavenka et al., 2006b;
Plašil et al., 2005]. These measurements demonstrated the high sensitivity of the spectrometer
that allowed us to observe the weak absorption transitions of species with low concentration,
such as ions in a cold plasma. As the time resolved cavity ringdown spectrometer (TR-CRDS)
has been described elsewhere [Macko et al., 2004; Hlavenka et al., 2006a; Hlavenka et al.,
2006b], only a short description will be given. The apparatus consists of the discharge tube
with two highly reflective mirrors on each end of the tube, making up an optical cavity. Laser
beam is produced by an external cavity diode laser (SacherLaser TEC 500). After spatial
matching the beam is injected into the optical cavity. The incident beam is interrupted (using
an acousto-optic modulator) after sufficient build-up of the intensity in the cavity is detected,
and a photodiode detector then records the light intensity decay caused by losses on mirrors
‰
98
Absorbance [dimensionless]
HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY
OH
3x10-8
H
H
-8
2x10
O+
O+
3
3
H
H
O+
3
O
2
1x10-8
0
2x10-8
H
Discharge
O+
3
0.7-3.3 ms
.5-7 ms
Background 4
1x10-8
0
6911.5
6912.0
6912.5
W avenumber [cm
6913.0
-1
]
Figure 2. The absorption spectra obtained by CRDS using synchronous detection. Upper panel
shows the absorbance (the relative loss of light intensity per pass) in discharge and background
time windows. One can clearly see the elevation of baseline in case of OH line. For H2 O, the
both curves are the some. Lower panel shows absorbance spectrum with subtracted baseline.
and due to the absorption of species inside the optical cavity. From each exponential decay
(called ringdown) the absorption can be directly determined. The typical ringdown time was
∼33 µs for a 75 cm long empty cavity.
To increase the sensitivity and to simplify the assignment of the observed absorption lines,
synchronous detection scheme has been applied [Hlavenka et al., 2006a; Hlavenka et al., 2006b].
Briefly, for each wavelength of the laser the time resolved absorption measurements are obtained
from many ringdown events over many discharge periods. Than, two detection windows are
selected from this time series; those corresponding to the discharge and those corresponding to
the end of the afterglow, where the microwaves are off and the ion concentration is negligible.
By subtracting the average signals in those two windows the net ion absorption is determined.
The laser wavelength was monitored using a Michelson interferometer based wavemeter (absolute calibration) and Fabry-Perot etalon (relative wavelength change), for details see [Hlavenka
et al., 2006a; Hlavenka et al., 2006b]. The observed water transitions served to check the calibration reliability. The error of wavelength calibration is ∼0.01 cm−1 .
Results
The first OH stretching overtone region of H3 O+ ion in NIR was scanned with CRD spectrometer in the region 6900 - 6960 cm−1 with approximately 80% coverage. The remaining
∼20% of this region could not be presently reached due to the gaps in the tuning characteristics
of the laser. An example of obtained spectra is plotted in Fig 2. The upper line is the spectrum
corresponding to the ’discharge ON’ window and the lower line (slightly shifted downwards
for clarity) is the spectrum obtained during the late afterglow (’discharge OFF’), as described
above. It can be seen that indeed certain spectral lines are observed with virtually the same
intensity both in the active discharge and in the late afterglow, indicating that they belong
to stable species, while others exhibit vastly different intensities in the active discharge and
afterglow, indicating they belong to discharge generated species. It is also however shown, that
there are clear differences in the observed ON/OFF characteristics for different observed lines.
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HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY
3x10-8
-8
2x10
O+
H
3
6948.93 cm-1
Absorbance evolution
on peak center
Absorbance [dimensionless]
1x10-8
0
6x10-8
OH
6956.40 cm-1
-8
x10
4
2x10-8
0
6x10-7
-7
H
O
2
6956.32 cm-1
x10
4
Microwave intensity
[a.u.]
2x10-7
0
-0.04 -0.02 0.00 0.02 0.04 0.06
W avenumber [cm
-1
0
2
4
6
8
10
12
Time [ms]
]
Figure 3. Demonstration of the synchronous detection used to distinguish between different
species. Left panels show the absorbance spectrum during 1-4 ms time window in active discharge (full line) and spectrum during 6-9 ms time window in the far afterglow (dashed line).
Right panels show time evolution of absorbance at central wavelength of the peak. The microwave intensity evolution is depicted in lower panel. The electronics of the microwave source
was not able to switch off the power rapidly, so at the falling edge both MW and H3 O+ signal
falls slower compared to the edge of the switching pulse.
Thus while this two-window-subtraction synchronous detection (as described above) is very
useful for the initial data characterization, the details of the time evolution obtained at each
laser wavelengths must be used to distinguish in more detail between the stable (H2 O), the
long-lived species (such as the OH radical), and the short lived ionic species such as the H3 O+ .
This is clearly demonstrated in Fig. 3 that shows the absorption signal time evolution for the
three representative spectral lines. The H3 O+ average lifetime after switching off the discharge
is primarily limited by the recombination with electrons and is much shorter compared to the
lifetime of neutral species generated in plasma, such as the OH radical (that are removed mainly
on the walls). Still the concentration of those species is rather dramatically affected by the
discharge. This is in clear contrast to the stable species, as demonstrated in the third panel of
Fig. 3 for representative H2 O absorption line. If one puts the ’discharge OFF’ detection window
closer to the end of active discharge, where the concentration of H3 O+ is already negligible, but
the OH radical concentration is still observable, one can see the background (baseline) elevation
in the scan in case of a OH radical spectral line, as demonstrated on the left panel of Fig. 3.
In case of H2 O lines, both signal and background spectra are identical. This approach can give
us a clue, how the H3 O+ transitions can be distinguished from the transitions of other species.
However, only a proper line analysis and line assignment can validate, weather a line belongs
to the H3O+ band or to an unidentified short living excited molecule or ion. Although the full
analysis and assignment haven’t been completed yet, the preliminary analysis identified more
than 30 lines as H3 O+ , more than 40 as H2 O and more than 15 as OH radical or other neutral
metastable molecule.
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HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY
Conclusion
The position of OH stretching overtone band of H3 O+ ion has been estimated. The region
6900 - 6960 cm−1 have been scanned with high sensitivity and resolution using CRDS spectrometer. For higher sensitivity the concentration modulation technique have been used together
with time resolved measurement. It was demonstrated, that this technique is essential in this
kind of experiments to distinguish the short-lived H3 O+ ions from metastable radicals (OH)
and stable molecules (H2 O). The recorded H3 O+ lines will serve after assignment as a valuable
benchmark for theoretical prediction done for H3 O+ and NH3 , and can be used for corrections
of the multidimensional potential energy surface (PES) of H3 O+ at higher energies.
Acknowledgments. This work is a part of the research plan MSM 0021620834 that are financed by the Ministry of Education of the Czech Republic and partly was supported by Kontakt
CZ-8/2004, GACR (202/03/H162, 205/05/0390, 202/05/P095 and 202/03/0827) and GAUK-278/2004
/B-FYZ/MFF.
References
Begemann M. H., C. S. Gudeman, J. Pfaff, and R. J. Saykally, Detection of the Hydronium Ion (H3 O+ )
by High-Resolution Infrared Sepctroscopy, Phys. Rev. Lett. 51, 554., 1983.
Begemann M. H., R. J. Saykally, A study of the structure and dynamics of the hydronium ion by
+
high-resolution infrared laser spectroscopy. I. The ν3 band of H16
3 O , J. Chem. Phys. 82, 3570, 1985.
Bunker P. R., T. Amano, and V. Spirko, A Preliminary Determination of the Equilibrium Geometry and
Inversion Potential in H3 O+ from Experiment, J. Mol. Spectr. 107, 208, 1984.
Davies P. B., S. A. Johnson, P. A. Hamilton, and T. J. Sears, Infrared diode laser spectroscopy of the
ν2 (2+ ← 1− ) band of H3 O+ , Chem. Phys. 108, 335, 1986.
Gruebele M., M. Polak, and R. J. Saykally, A study of the structure and dynamics of the hydronium ion
+
by high resolution infrared laser spectroscopy. II. The ν4 perpendicular bending mode of H16
3 O , J.
Chem. Phys. 87, 3347, 1987.
Haese N. N. and T. Oka, Observation of the ν2 (1− ← 0+ ) inversion mode band in H3 O+ by high resolution
infrared spectroscopy, J. Chem. Phys. 80, 572, 1984.
Hlavenka P., R. Plašil, G. Bánó, I. Korolov, D. Gerlich, J. Ramanlal, J. Tennyson, J. Glosı́k, Near
infrared Second overtone cw-Cavity Ringdown spectroscopy of D2 H+ ions, Int. J. Mass Spect. 2006a,
in press.
Hlavenka P., I. Korolov, R. Plašil, J. Varju, T. Kotrı́k, J. Glosı́k, Near infrared Second overtone cw-Cavity
Ringdown spectroscopy of H2 D+ ions, Czechoslovak J. of Phys. Suppl. B 56, 2006b.
Huang X., S. Carter, and J. M. Bowman, Ab Initio Potential Energy Surface and Vibrational Energies
of H3 O+ and Its Isotopes, J. Phys. Chem. B 106, 8182, 2002.
Liu D. and T. Oka, Experimental Determination of the Ground-State Inversion Splitting in H3 O+ , Phys.
Rev. Lett. 54, 1787, 1985.
Lundsberg-Nielsen L., F. Hegelund, and F. M. Nicolaisen, Analysis of the High-Resolution Spectrum of
Ammonia (14 N H3 ) in the Near-Infrared Region, 6400-6900 cm−1 , J. Mol. Sp. 162, 230, 1993.
Macko P., G. Bánó, P. Hlavenka, R. Plašil, V. Poterya, A. Pysanenko, O. Votava, R. Johnsen and
J. Glosı́k, Afterglow studies of H+
3 (ν = 0) recombination using time resolved cw-diode laser cavity
ring-down spectroscopy, Int. J. Mass Spect. 233, 299, 2004.
Plašil R., P. Hlavenka, P. Macko, G. Bánó, A. Pysanenko and J. Glosı́k, The recombination of spectroscopically identified H+
3 (ν = 0) ions with electrons, J. Phys. Conf. Series 4 118, 2005.
Rajamaki T., A. Miani, and L. Halonen, Six-dimensional ab-initio potential energy surfaces for H3 O+ and
NH3 : Approaching the subwave number accuracy for the inversion splittings, J. Chem. Phys. 118,
10929, 2003.
Spirko V. and W. P. Kraemer, Anharmonic Potential Function and Rotation-Inversion Energy Levels of
H3 O+ , J. Mol. Spectr.134, 72, 1989.
Tang J. and T. Oka, Infrared Spectroscopy of H3 O+ : The ν1 Fundamental band, J. Mol. Spectr. 196,
120, 1999.
Uy D., E. T. White, and T. Oka, Observation of ∆|k-I| = 3 Transitions in the ν3 Band of H3 O+ , J. Mol.
Spectr. 183, 240, 1997.
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