Download LIST OF FIGURES 1.1 Molecular orbitals, their approximate energy

LIST OF FIGURES
1.1
Molecular orbitals, their approximate energy levels and type of
transitions in formaldehyde molecule. In the formation of >C=O
bond, the six electrons, four from oxygen and two from carbon, are
accommodated in the three lower energy levels of σ, π and n.
4
1.2
The Jablonski diagram, showing different photo-physical
processes undergone by an excited state molecule in condensed
phase. Straight and wavy arrows represent radiative and nonradiative
processes respectively. The spin of electrons in
each of the singlet states (paired spin) compared to the triplet state
(unpaired spin).
6
1.3
Energy levels of molecular excited states and transitions between
them. Figure also depicts a pathway for dissociation of
electronically excited species. Here population prepared in S1 state
is transferred to T1 state, which is well above its dissociation
threshold and thus dissociates to form photofragments.
7
1.4
Schematic drawings of ground and electronically excited potential
energy surfaces illustrating VMP [a] directly dissociative excited
states; [b] predissociative excited state.
12
1.5
Photodissociation processes of diatomic molecules with initial
internal energy. [a] Direct dissociation. [b] Electronic
predissociation in which molecule undergoes radiationless
transition (rt) from bound to the repulsive state and subsequently
decays. [c] Vibrational predissociation in which the photon creates
a quasi-bound state in the potential well which decays either by
tunneling (tn) or by intramolecular vibrational energy
redistribution (IVR). [d] Spontaneous radiative dissociation. Ei is
the energy of the parent molecule.
13
1.6
The shift in mean speed and the width of the distribution brought
about by use of a supersonic nozzle.
17
1.7
A supersonic nozzle skims off some of the molecules of the beam
and leads to a beam with well defined velocity.
18
1.8
Cartoon of H2 elimination from 1,4-cyclohexadiene showing a side
and a top view of the transition state region and the helicopter
motion of H2 product.
23
1.9
Schematic diagram of
Fluorescence technique.
Induced
28
1.10 Schematic of some MPI schemes: a) (1+1)REMPI; b)
(2+1)REMPI; and c) (3+1)REMPI. Other schemes are also
possible. Si denotes ground state; Sk denotes resonant excited state;
30
Laser
xxvi Photolysis-Laser
IP denotes ionization continuum.
1.11 Schematic of the photofragment imaging apparatus. A pulsed
molecular beam enters the photodissociation region of the
apparatus through a hole in a repeller plate of a time of flight mass
spectrometer. A photolysis laser beam intersects the molecular
beam and causes photodissociation. A few nanoseconds later a
tunable UV laser beam intersects the fragments and resonantly
ionizes them. A few microseconds later the ion impacts a position
sensitive ion detector creating a two-dimensional projection of the
ion distribution. The image is recorded in CCD camera. In order to
obtain REMPI spectra a photomultiplier is used to monitor the
detector
32
1.12 CARS energy scheme.v1and v0 are ground state vibrational levels.
ωp, ωs, ωpr and ωCARS are frequencies of pump, stokes, probe and
CARS signal beams.
33
1.13 The Schematic of the operating principle of Cavity Ring-Down
Spectrocsopy.
34
1.14 The position of the barrier controls what type of energy is
deposited into productdegrees of freedom as well as what type of
reactant energy is required.
37
1.15 A hypothetical model of a molecule, where the γ−δ bond breaks
impulsively.
44
1.16 Schematic diagram illustrating the relations between various
energetic quantities in the hybrid model (barrier impulsive model).
46
1.17 Sequence of execution of program modules resulting in the
optimization of the molecular geometry for a particular basis set.
54
2.1
Schematic diagram of LP-LIF set-up.
63
2.2
Schematic diagram of MB-REMPI-TOF-MS technique.
64
2.3
A partial rotational energy level diagram of the ground X 2Π
(ν″=0) and first excited A2Σ+ (ν′=0) electronic states of OH
including some dipole allowed transitions. The energy levels are
labeled by both quantum numbers N and J, parity p and Λdoubling symmetry e/f. The energy splitting of the Λ-doublet and
ρ-doublet components is exaggerated for reasons of clarity.
69
2.4
An experimental set-up for laser photolysis-laser induced
fluorescence technique.
71
2.5
The schematic for solid samples.
73
xxvii 2.6
REMPI scheme for the chlorine atom for the two spin orbit states.
82
2.7
REMPI scheme for the bromine atom for the two spin orbit states.
82
2.8
A schematic diagram of MB-REMPI-TOF-MS system.
83
2.9
The resultant output wavelength around 235 nm from Quantel dye
laser with frequency doubling and mixing modules pumped by
Nd:YAG laser.
87
2.10 Illustrates change in time of flight profile (at the bottom of the
panel) by the correlation of the polarization of dissociating laser
light when the molecular transition dipole moment is aligning
either parallel (left panel) or perpendicular (right panel) to the
recoil velocity of photofragment.
92
2.11 Energy levels of Nd3+:YAG.
95
2.12 Schematic energy level scheme and pumping cycle in dye
molecules.
97
2.13 Schematic potential energy diagram of an excimer laser.
100
2.14 Schematic of the vacuum system employed for LP-LIF setup.
105
2.15 Schematic of the vacuum system employed for REMPI setup.
106
2.16 The external appearance of (a) Side-on Type and (b) Schematic of
working of photocathode with reflection mode.
2.17 Plot of arrival time (μs) for various fragments (m/z) for TCE for
mass calibration.
107
3.1
A typical LIF excitation spectrum of the (0, 0) band of the
A2Σ+←X2Π system of the nascent OH radical formed in
photodissociation of cyclohexanone oxime (50 mTorr) at 193 nm.
The time delay between pump and a probe laser was 50 ns. The
rotational lines are assigned in the figure.
120
3.2
Boltzmann plots of rotational state population against energy of
rotational states of OH (v″= 0) generated in dissociation of CHO
(open circles) and CPO (solid circles) with 193 nm laser.
121
3.3
Doppler profile of P1(2) line of the A2Σ+←X2Π (0,0) system of
the OH radical produced in dissociation of CHO (open circles) and
CPO (solid circles) with 193 nm laser. The dotted line shows the
laser spectral profile.
122
3.4
Dependence of ratio of Λ-doublet (denoted by open circles) and
spin-orbit state (denoted by filled circles) populations against
rotational quantum number N” for the nascent OH formed in laser-
124
xxviii 111
induced photodissociation of CHO (a) and CPO (b) at 193 nm.
3.5
Time evolution of OH (v″= 0), in different rotational levels (N″),
from CHO on excitation at 193 nm. The plot shows the LIF signal
for N″=2, 3, 4, 5 and 6 with time delay.
125
3.6
The optimized structures of the ground electronic state (most
stable anti conformers depicted as CHO and CPO), the T2 state
(marked as CHO_T2 and CPO_T2) and the transition states
(marked as CHO_TS and CPO_TS) from the T2 state for OH
formation from CHO (shown in the left column) and CPO (shown
in the right column). A few important bond distances (in Å) and
dihedral angles (only for CHO, in degrees) are given in the figure.
The ground and the excited electronic state structures are
optimized at the B3LYP/6-311++G(d,p) and CIS/3-21G level of
theory, respectively.
128
3.7
The schematic energy diagram for formation of OH fragment after
photodissociation of CPO at 193nm. The figure shows two isomers
of CPO, CPO_anti and CPO_syn, and the transition state of
conversion of these two forms. The OH fragment formation takes
place from T2 state and the transition state involved in it is also
shown. The scheme for CHO is same as that for CPO, the energy
values given in green colour are for CHO molecule.
138
4.1
A portion of typical LIF spectrum of OH after photodissociation of
CHD (10 mTorr) at 248 nm and the pump-probe delay of ~50 ns.
144
4.2
Boltzmann plots of rotational distributions of the nascent OH
radical generated on A) 266 nm and B) 248 nm photolysis. The
solid lines are the fit to the experimental data points.
145
4.3
The statistically weighted spin-orbit ratios of nascent OH (v‫=״‬0) as
a function of the rotational quantum number (N). The red filled
squares and the black filled circles denote the ratios at 266 and 248
nm photolysis, respectively.
146
4.4
Λ-doublet ratio of nascent OH(v‫=״‬0) as a function of the rotational
quantum number (N). The red filled squares and the black filled
circles denote the ratios at 266 and 248 nm photolysis,
respectively.
147
4.5
Doppler profile of the P1 (4) line of the (0, 0) band of the A–X
system of OH on 248 nm photodissociation of CHD.
149
4.6
Optimized structures of three conformer of 1,2-cyclohexanedione.
150
4.7
Computed MOs involved in the transition of both the conformers
of enolic 1,2–Cyclohexanedione (CHD).
151
xxix 4.8
Different optimized structures for various excited states of CHD
(for details see the text).
152
4.9
Potential energy curves for various excited electronic states of
different CHD structures namely H–bonded (a) and non–H–
bonded structures (c) calculated with the TD-DFT method as a
function of the C2–O2 bond length.
165
4.10 The schematic energy diagram for CHD molecule. The figure
shows the three isomers of CHD namely, H-bonded, non-Hbonded and keto-form of CHD and the transition state for
conversion from non-H-bonded form to H-bonded form. The
energetic of T1, T2, S1 and S2 states formed after photodissociation
of both non-H-bonded and H-bonded CHD at 193, 248 and 266
nm are also shown in the figure.
169
5.1
The fluorescence excitation spectra of the (0,0) band of the
A2Σ+−Χ2Π system of OH formed on photolysis of ClNT at 193
nm.
174
5.2
The fluorescence excitation spectra of the (1,1) band of the
A2Σ+−Χ2Π system of OH formed on photolysis of NCP at 193 nm.
175
5.3
Typical Boltzmann plots of the distribution of rotational energy in
the nascent OH from photolysis of ClNT 193, 248 nm, 266 nm in
v″=0 vibrational level.
176
5.4
Typical Boltzmann plots of the distribution of rotational energy in
the nascent OH from photolysis of NCP in (a) in v″=0
vibrational level, and (b) in v″=1 vibrational level.
177
5.5
(a) Doppler profiles of P1(5) line of the A2Σ+ ← X2Π (0,0) system
of the OH radical produced in dissociation of ClNT at 193, 248
and 266 nm laser.
b) Doppler profile of P1(5) line of the A2Σ+ ← X2Π (0,0) system of
the OH radical produced in dissociation of NCP at 193 nm laser.
179
5.6
(a)The statistically weighted spin-orbit ratios of nascent OH (v″=0)
as a function of the rotational quantum number (N). The black,
blue, and red circles denote the ratios at 266, 248 and 193 nm
photolysis, respectively for ClNT.
(b) The statistically weighted spin-orbit ratios of nascent OH
(v″=0) as a function of the rotational quantum number (N).The
black filled circles for NCP.
180
xxx 181
5.7
(a) Λ-doublet ratio, Π– (A")/ Π+ (A'), of nascent OH(v″=0) as a
function of the rotational quantum number (N). The black, blue
and red filled circles denote the ratios at 266 248 and 193 nm
photolysis, respectively, for ClNT.
(b) Λ-doublet ratio, Π– (A")/ Π+ (A'), of nascent OH(v″=0) as a
function of the rotational quantum number (N). The black filled
circles for NCP at 193 nm.
182
5.8
(a) Fluorescence spectra recorded 200 ns after the photolysis of
NCP.
(b) Variation of emission signal at 540 (black) and 310 (red) nm
with intensity of the photolyzing laser (193 nm).
183
5.9
The measured UV absorption spectra for (a) ClNT and (b) NCP in
gas phase.
185
5.10 The schematic energy diagram of OH fragment formation
pathways on excitation of NCP at 193 nm and ClNT at 193, 248
and 266 nm. For both the molecules the direct OH elimination
channel is lower energy channel than the formation of OH
fragment via HONO molecule elimination.
196
6.1
Optimized geometries for various isomers of fumaryl chloride.
200
6.2
Various REMPI lines for Chlorine atom formed during the
photolysis of tetrachloroethylene.
201
6.3
Dependence of the observed Cl(2P3/2) atom REMPI signal from
fumaryl chloride photolysis on the laser intensity. The slope of the
fitted linear log–log plot is 3.1 ± 0.2.
203
6.4
Profiles of Cl and Cl* atoms produced in the 235 nm laser
photolysis of fumaryl chloride used for the determination of their
relative quantum yields.
204
6.5
REMPI-TOF profiles of [a] Cl(2P3/2) and [b] Cl(2P1/2) produced
from the 235 nm photodissociation of fumaryl chloride. The
circles are the experimental data and the solid line is a forward
convolution fit. Three panels, namely, upper, middle and lower
panels correspond to horizontal, magic angle, vertical,
experimental geometries, respectively.
206
6.6
Centre-of-mass recoil translational energy distribution derived
from FIG. 6.5 for [a] Cl (2P3/2) and [b] Cl*(2P1/2) in the
photodissociation of fumaryl chloride at 235 nm. The red lines
indicate the speed distributions for the fast and slow component
for chlorine atom formation channel, respectively; the solid line
shows the sum. The vertical arrow indicates the maximum
available energy for the respective chlorine, Cl (2P3/2) or
Cl*(2P1/2), elimination channel.
207
xxxi 7.1
Various REMPI lines for bromine atom formed during the
photolysis of bromoform.
221
7.2
Dependence of the REMPI signal of Br(2P3/2) atom from 2-bromo5-chlorothipohene photolysis on the laser intensity. The slope of
the fitted linear log-log plot is 3.3±0.2.
223
7.3
REMPI spectral profiles of Cl and Cl* atoms produced in
photodissociation of 2-chlorothiophene at 235 nm, used for the
determination of their relative quantum yields.
225
7.4
REMPI spectral profiles produced in photodissociation of 2bromo-5-chlorothiophene at 235 nm; (A) Cl and Cl* and (B) Br
and Br*, used for the determination of their ratio.
226
7.4
REMPI–TOF profiles; (A) Cl (2P3/2) and Cl* (2P1/2) produced from
the photodissociation of 2-chlorothiophene: (B) Cl (2P3/2) and Cl*
(2P1/2) and (C) Br (2P3/2) and Br* (2P1/2) produced from the
photodissociation of 2-bromo-5-chlorothiophene at 235 nm. The
circles are the experimental data and the solid curves are forward
convolution fit. Velocity components are depicted as dotted
curves. The profiles correspond to the magic angle (χ=54.7°)
experimental geometry.
227
7.6
Centre-of-mass recoil translational energy distribution derived
from FIG. 7.5: (A) Cl(2P3/2) and Cl* (2P1/2), produced in the
photodissociation of 2-chlorothiophene, (B) and (C) Cl(2P3/2) and
Cl* (2P1/2) and Br (2P3/2) and Br* (2P1/2), produced in the
photodissociation of 2-bromo-5-chlorothiophene, respectively, at
235 nm. Here, in (A) the dashed lines indicate the translational
energy distributions for the fast and slow components for chlorine
atom formation channel, the solid line shows the sum. The blue
and red vertical arrows indicate the maximum available energy for
the Cl (2P3/2) and Cl* (2P1/2) or Br (2P3/2) and Br* (2P1/2)
elimination channels, respectively.
229
7.7
Relative energy diagram in kcal/mol, along with the structures, for
various products in the photodissociation of 2-chlorothiophene at
235 nm in its ground state.
234
7.8
Computed HOMO, LUMO, along with other MOs involved in the
transition of thiophene, 2-chlorothiophene, and 2-bromo-5chlorothiophene at 235 nm.
239
xxxii