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Experimental investigations of IR spectra of highly vibrationally excited molecules
Spectrochimica ACM, Vol. 43A. Printed in Great Bntam.
No.
2, pp. 167-168,
EXPERIMENTAL VIBRATIONALLY
0X34-8539/87 $3.OO+O.M) Pergamon Journals Ltd.
1987.
INVESTIGATIONS
OF IR SPECTRA OF HIGHLY
EXCITED MOLECULES
S.I. Ionov Research Centre for Laser Technology Laboratory of Laser Chemistry 142092
Troitzk, Moscow Region, USSR
considerable attention is directed to investigation
Today
molecular vibrational IR spectroscopy
relaxation
(IVR) /l/in
ofintra-
highly excited molecules.
is one of the ways to study such processes. It is due to
the fact, that dynamics of a system is closely connected with its spectroscopy. Here, the results of IR spectroscopic
studies of the (CF3)3CI and
CF31 molecules are announced/2-4/. The vibrational energies of the molecules under investigation are Ei>,Di, where Di are the dissociation thresholds. The IR spectra were obtained by means of the photodissociation nique/2/.
tech-
The main idea of this technique is the following. Let a distribu-
tion of highly vibrationally molecules with E>
excited molecules
is created by any way. The
D decay with rates k(E). Affect this molecular
ensemble
by a weak probe IR pulse and then, after delay t measure the dissociation It can be shown/2/,
yield -9.
enhanced product yield -A3 having the E* vibrational
that in the case of weak probe fluence @
pr
is due to IR absorption of the molecules energy, where E* can be determined from the rela-
tion k(E*)tr\, 1; the value of a? /s,r at 10w&~~ the absorption cross-section GE, of the molecules range 4 E near the E*. Thus far,
being proportional
to
from a narrow energy
measurements of the spectral dependence
give an IR spectrum of highly excited molecules, the vibraof 3Jj/Bpr tional inhomogeneous broadening of the spectrum being insignificant in such experiment. In the course of experiment the beam of the (CF3)3CI molecules was succesively irradiated by two IR and a UV laser pulses. The IR pulses were generated by TEA CO2 lasers. The first pulse carried out IR multiple photon excitation of the molecules.
The second IR pulse was the probe
in the course of spectral measurements.
The W
a dye laser radiation) produced photoionization the product tion
as
pulse (the 2nd harmonic of detection of atomic I -
of the parent molecules decay. For this reason the UV radia-
tuned
to the 2-photon resonance 5p2P1,2 - 6p4P1,2
(A
= 2982.3
II)
of ionization of the atom. Delay between the pump and probe IR pulses was equal to 5fis,
delay between the probe IR and UV pulses was equal to 10~s.
We measured the fluence dependences
of the dissociation yield $(a
different frequencies 1/ of the IR probe radiation. Experimental well fitted straight lines at low @,,. mentioned,
is proportional
The slope. -aP/a@
to the IR absorption 167
) at poP:ts
, as has been cross-sections pr KE,(J ).
168
s. I. IONO\l
Figurlwl.
'GOV,cm-I
Figure 2.
The (CF3)3CI spectrum obtained /3/ (see Fig. L) turns out 25 cm" shifted to the red from the harmonic frequency (v,, = 958 cm-') because of anharmonicity. Experimental data fit well the Lorentzian (curve 1) with the halfwidth v = 9 cm-' in the spectral range of -56 7/ -V”$ t4 . From the r obtained one can estimate the IVR rate TI,R>, 0.3 ps. The similar experiment has been done with the small molecule - CF31/4/ (see Fig. 2). The CF I IR spectrum is 40 cm-' shifted to the red from the 3h harmonic frequency (V, = 1075 cm-'). Contrary to the spectrum of the large molecule - (CF3j3CI, they , spectrum of CF31 has distinct structure, namely, it consists of one central and two lateral bands. (We believe, that the rise of absorption in the region of v7 1060 c..-'is due to another mode of the molecule - 1/ 4. It harmonic frequency V 4 = 1175 cm-'). The lateral bands are -58 cm-' and +15 cm-' apart from the central one. These values coincide well with the positions of the nearest Fermi-resonances in the cold molecule: 7), - (v, t y3) = -55 cm-'; 1/, - 2u5 = +I0 cm-1 (see Fig. 2b). We attribute the lateral bands to v 2 t v, and 2v5. Note, that the adifference of the CF31 spectrum from the Lorentzian points out a nonexponential relaxation of energy from the7/, mode of the molecule. The difference between the (CF3)3CI and CF31 spectra arises from large distinction of the Fermi-resonances densities of the molecules (~0.3Res,& anda0.05 Res/cm-I, respectively). Particular resonances cover each other in the large molecule and smooth spectrum is observed. In the case of the small molecule rear resonances are not covered. It leads to the structure of the spectrum and to nonexponential relaxation of energy from the mode.
References 1. Intramolecular Dynamics, Ed. by J. Jortner and B.Pullman (Reidel, New-York, 1982). 2. S.I. Ionov, to be published. 3. V.N. Bagratashvili, S.I. Ionov, V.S. Letokhov, V.N. Lokhman, A.A. Stuchebrukhov, to be published. 4. V.N. Bagratashvili, 0.1. Bojarkin, S.I. Ionov, to be published.
No.
2, pp. 167-168,
EXPERIMENTAL VIBRATIONALLY
0X34-8539/87 $3.OO+O.M) Pergamon Journals Ltd.
1987.
INVESTIGATIONS
OF IR SPECTRA OF HIGHLY
EXCITED MOLECULES
S.I. Ionov Research Centre for Laser Technology Laboratory of Laser Chemistry 142092
Troitzk, Moscow Region, USSR
considerable attention is directed to investigation
Today
molecular vibrational IR spectroscopy
relaxation
(IVR) /l/in
ofintra-
highly excited molecules.
is one of the ways to study such processes. It is due to
the fact, that dynamics of a system is closely connected with its spectroscopy. Here, the results of IR spectroscopic
studies of the (CF3)3CI and
CF31 molecules are announced/2-4/. The vibrational energies of the molecules under investigation are Ei>,Di, where Di are the dissociation thresholds. The IR spectra were obtained by means of the photodissociation nique/2/.
tech-
The main idea of this technique is the following. Let a distribu-
tion of highly vibrationally molecules with E>
excited molecules
is created by any way. The
D decay with rates k(E). Affect this molecular
ensemble
by a weak probe IR pulse and then, after delay t measure the dissociation It can be shown/2/,
yield -9.
enhanced product yield -A3 having the E* vibrational
that in the case of weak probe fluence @
pr
is due to IR absorption of the molecules energy, where E* can be determined from the rela-
tion k(E*)tr\, 1; the value of a? /s,r at 10w&~~ the absorption cross-section GE, of the molecules range 4 E near the E*. Thus far,
being proportional
to
from a narrow energy
measurements of the spectral dependence
give an IR spectrum of highly excited molecules, the vibraof 3Jj/Bpr tional inhomogeneous broadening of the spectrum being insignificant in such experiment. In the course of experiment the beam of the (CF3)3CI molecules was succesively irradiated by two IR and a UV laser pulses. The IR pulses were generated by TEA CO2 lasers. The first pulse carried out IR multiple photon excitation of the molecules.
The second IR pulse was the probe
in the course of spectral measurements.
The W
a dye laser radiation) produced photoionization the product tion
as
pulse (the 2nd harmonic of detection of atomic I -
of the parent molecules decay. For this reason the UV radia-
tuned
to the 2-photon resonance 5p2P1,2 - 6p4P1,2
(A
= 2982.3
II)
of ionization of the atom. Delay between the pump and probe IR pulses was equal to 5fis,
delay between the probe IR and UV pulses was equal to 10~s.
We measured the fluence dependences
of the dissociation yield $(a
different frequencies 1/ of the IR probe radiation. Experimental well fitted straight lines at low @,,. mentioned,
is proportional
The slope. -aP/a@
to the IR absorption 167
) at poP:ts
, as has been cross-sections pr KE,(J ).
168
s. I. IONO\l
Figurlwl.
'GOV,cm-I
Figure 2.
The (CF3)3CI spectrum obtained /3/ (see Fig. L) turns out 25 cm" shifted to the red from the harmonic frequency (v,, = 958 cm-') because of anharmonicity. Experimental data fit well the Lorentzian (curve 1) with the halfwidth v = 9 cm-' in the spectral range of -56 7/ -V”$ t4 . From the r obtained one can estimate the IVR rate TI,R>, 0.3 ps. The similar experiment has been done with the small molecule - CF31/4/ (see Fig. 2). The CF I IR spectrum is 40 cm-' shifted to the red from the 3h harmonic frequency (V, = 1075 cm-'). Contrary to the spectrum of the large molecule - (CF3j3CI, they , spectrum of CF31 has distinct structure, namely, it consists of one central and two lateral bands. (We believe, that the rise of absorption in the region of v7 1060 c..-'is due to another mode of the molecule - 1/ 4. It harmonic frequency V 4 = 1175 cm-'). The lateral bands are -58 cm-' and +15 cm-' apart from the central one. These values coincide well with the positions of the nearest Fermi-resonances in the cold molecule: 7), - (v, t y3) = -55 cm-'; 1/, - 2u5 = +I0 cm-1 (see Fig. 2b). We attribute the lateral bands to v 2 t v, and 2v5. Note, that the adifference of the CF31 spectrum from the Lorentzian points out a nonexponential relaxation of energy from the7/, mode of the molecule. The difference between the (CF3)3CI and CF31 spectra arises from large distinction of the Fermi-resonances densities of the molecules (~0.3Res,& anda0.05 Res/cm-I, respectively). Particular resonances cover each other in the large molecule and smooth spectrum is observed. In the case of the small molecule rear resonances are not covered. It leads to the structure of the spectrum and to nonexponential relaxation of energy from the mode.
References 1. Intramolecular Dynamics, Ed. by J. Jortner and B.Pullman (Reidel, New-York, 1982). 2. S.I. Ionov, to be published. 3. V.N. Bagratashvili, S.I. Ionov, V.S. Letokhov, V.N. Lokhman, A.A. Stuchebrukhov, to be published. 4. V.N. Bagratashvili, 0.1. Bojarkin, S.I. Ionov, to be published.