This paper presents an overview of data obtained on the intramolecular vibrational
energy distribution in infrared multiphoton excited CF2HCl, CF2Cl2, SF6 and CH3CHF2. All but CF2HCl show collisionless changes in the intensity of the spontaneous Raman signals after excitation, indicating that the excitation alters the population in the Raman active modes. A comparison of the spectrally integrated intensities of the Raman signals yields information on the distribution of vibrational energy over the modes of the molecule. The results for CF2Cl2 show a nonthermal distribution of energy after the excitation.

Jyhpyng  Wang

Kuei-Hsien  Chen

Eric  Mazur

Raman spectroscopy of infrared multiphoton excited molecules

Low pressure (53 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>&#x03BC;</mml:mi></mml:math>) infrared multiple photon decomposition of several hydrocarbons requiring large fluences to 
produce measurable decomposition was investigated. Cyclopropane, propylene, methylcyclopropane, 
cis- and trans-2-butene, vinylcyclopropane, cyclopentene and 1-methylcyclopentene exhibit wide variations 
in reaction product yield and the spectral dependence of yield at constant pressure and fluence. The role 
vibrational state density, torsional vibrations and low intensity absorption cross section play in determining 
yields was examined. The results show the complex interplay of factors affecting multiple photon decomposition. 
Although the maximum observed yields tend to increase with increasing vibrational state density and absorption 
cross section, they were poorly correlated with either state density or cross section. The dependence of yield upon 
excitation wavenumber revealed several unexpected features which indicate that low energy vibrational level 
structure must be the dominant factor in determining yields. Also, the data suggest that torsional vibrations can 
ease the excitation bottleneck.

Murthy L. Yerram

Robert W. Carr

Comparisons of Infrared Multiple Photon Reaction Yields of Some C3&#8211;C6 Hydrocarbons

We have investigated the excitation of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mtext>N</mml:mtext><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>14</mml:mn></mml:mrow></mml:mmultiscripts><mml:msub><mml:mtext>H</mml:mtext><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> molecules (the pressure is 10&#8722;2 &#8722; 1 Torr) under the effect of pulse radiation of the 
CO2-laser in a wide range of fluences (&#x03A6; = 10&#8722;3&#8722;10 J/cm2) 
and detunings of the laser frequency from the absorption lines of the vibrational-rotational transition of the ammonia 
band &#957;2. It is shown that the efficient excitation of ammonia under the influence of nonresonance 
IR radiation is due to the power broadening. In this case the calculation, which employs no fitting parameters, 
describes the experimental results within experimental errors. It is shown that the abnormally high absorption 
of ammonia in the case of resonance pumping is associated with an intense FIR generation which leads to elimination 
of the rotational bottle neck under the excitation of molecules.

G. N. Makarov

M. V. Sotnikov

V. V. Tyakht

Nonlinear Absorption and Far-Infrared Generation Under the Excitation of Ammonia Molecules by Strong IR Radiation

The main channels for transformations of C2H5OH, 
			iso-CH3H7OH and n-C3H7OH molecules 
			under IR multiphoton excitation are the monomolecular reactions of dehydration (1), and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mtext>C</mml:mtext><mml:mi>&#x03B1;</mml:mi></mml:msub><mml:mo>&#x2212;</mml:mo><mml:msub><mml:mtext>C</mml:mtext><mml:mi>&#x03B2;</mml:mi></mml:msub></mml:mrow></mml:math>
 bond breaks (2). The ratio of the yields of reactions (1) and (2), &#947;, is determined by 
 the molecules&#39; structure, radiation energy density, the pressure of alcohol vapors and inert gas. 
 The direction of the alcohol molecules&#39; transformations under IR MPE, under thermal equilibrium conditions, 
 differs from pyrolysis, when the chain processes of C2H5OH and 
 C3H7OH molecules dehydrogenation, and CH4 splitting-out from 
 n-C3H7OH molecules are developing. Reactions of H and CH3 radicals 
 with alcohol molecules under laser initiation do not lead to the development of chains due to the high rate of 
 H atoms diffusion from reaction zone and effective
recombination of CH3 radicals.

G. P. Zhitneva

I. A. Markina

S. A. Pshezhetskii

Kinetics and Mechanism of Transformations of Aliphatic Alcohol Molecules Under Multiphoton IR Excitation

Infrared multiple-photon dissociation (IRMPD) of MoF6 has been investigated with a parallel
beam of a para-H2 Raman laser newly developed by our group. MoF6 gas 
(2 Torr, natural abundance) was put into in a static cell cooled to &#8722;58&#8451; and was irradiated with the laser line 
at 728 cm&#8722;1. The critical fluence of 0.11 Jcm&#8722;2 for complete dissociation was determined 
by analysis with a Gaussian beam model. Irradiation at the lower frequency side (728 cm&#8722;1) of 
the MoF6 &#957;3 band enriched the lighter isotopes in the residue. 
A maximum value for selectivity of 1.1 was obtained.

Toshiyuki  Oyama

Sakae  Satooka

Shuji  Kato

Katsumi  Midorikawa

Hideo  Tashiro

Kazuo  Takeuchi

Isotopically Selective Infrared Multiple-Photon Dissociation of
M<small.letters>o</small.letters>F6 by a H2-Raman Laser

We studied the IR luminescence spectra of vibrationally excited CF2Cl2 molecules
resulting from excitation of the &#957;1 (1098 cm&#8722;1) and 
&#957;8 (922 cm&#8722;1) modes with a pulsed CO2 laser. 
The nonequilibrium spectra obtained under pumping conditions where their equilibrium counterparts 
coincide (the number of the photons absorbed per molecule being the same) were found to differ considerably. 
We suppose that this difference is due to different types of vibrational distribution formed as a result of the IR laser pumping. 
When pumping the &#957;1 mode, excitation of the R-branch occurs, resulting in the 
molecules &#8220;sticking&#8221; on the lower vibrational levels, whereas in the case of the &#957;8 mode, 
it is the P-branch that gets excited so that the molecules become easy to raise to high-lying vibrational levels.

A. V. Dem&#39;Yanenko

G. A. Polyakov

A. A. Puretzky

IR-Luminescence of CF2Cl2 Molecules in Multiple-Photon Excitation With Co2-Laser Radiation

We present a very simple theoretical model aimed at the analysis of non-linear relaxation processes in 
molecular gases in the presence of partial molecular dissociation induced by vibrational&#8211;vibrational 
exchange between highly excited molecules. The model has a phenomenological character, since it analyzes the 
behavior of a system of anharmonic diatomic molecules, which is a very rough approximation of a polyatomic 
molecule such as SF6. Nonetheless, it provides an interesting key for the interpretation of a number 
of peculiar features characterizing our experimental observation, with which a comparison is made. In particular, 
the model takes realistic account of the influence of dissociation processes on the relaxation time.

L.  Carlomusto

A.  Cartelli

S.  Solimeno

R.  Velotta

R.  Bruzzese

Experimental and Theoretical Analysis of Non-linear Vibrational Relaxation of Polyatomic Molecules Strongly Excited by Resonant Laser Radiation

Pentafluoropyridine (PFP) undergoes fast structural isomerisation to fulvenes when irradiated 
			with 9R(16) CO2 laser line in the fluence range 0.5&#8211;1.5 J/cm2. 
			The unstable fulvenes slowly decay back to PFP in ms time scale. No detectable permanent dissociation of PFP 
			was observed in the above low fluence experiments. However, by using focused CO2 laser beam 
			for the irradiation of PFP, an emission of light in the visible/near-UV was observed. The time evolution of the 
			luminescence reveals three peaks at 390, 460 and 500 nm on a broad background emission of 300&#8211;680 nm. 
			These bands are assigned to the fluorescence of PFP via inverse electronic relaxation (IER) and C2 
			Swan bands. On prolonged irradiation of PFP in focused condition a small extent of permanent dissociation 
			was observed with the major products as C2F4, a sooty yellow deposit and another 
			compound presumed to be Dewar PFP.

P. K. Chowdhury

K. V. S. Rama Rao

J. P. Mittal

Transient Isomerisation and Inverse Electronic Relaxation of Infrared Multiple-Photon Excited
Pentafluoropyridine

The three-frequency IR excitation of visible luminescence was used as the method for probing of the 
 IR multiphoton absorption of OsO4 molecules cooled in a supersonic jet. The first laser pulse 
 was used to form a certain vibrational distribution below the onset of vibrational quasicontinuum. 
 The possibility of direct excitation of the vibrational states &#957; &#61; 2, 3 of pumped &#957;3 mode 
 by direct two- or three-photon absorption has been shown. The excitation spectra under the influence of the 
 second variable laser frequency show the features attained for transitions from initially prepared vibrational 
 levels &#957; &#61; 2 or &#957; &#61; 3. The dependencies of the luminescence intensity on the IR laser fluence were 
 studied also to prove the role of the direct multiphoton transitions. The different multiphoton and multifrequency 
 resonances were analysed in the frame of simple spectroscopic model, and appropriate anharmonicity constants 
 were derived.

A. V. Evseev

Multiphoton and Multifrequency Resonances in the IR Laser Excitation of O<small.letters>s</small.letters>O4 Molecules Cooled in a Supersonic Jet

The infrared multiphoton excitation of D2CO was studied by monitoring infrared fluorescence from stretching modes of D2CO following excitation in the bending modes. The dependence of the infrared multiphoton dissociation yield on laser wavelength, on laser power, and on reactant and foreign gas pressures was also studied. Severe bottlenecks to excitation through the first few steps are found. Vibration-to-vibration transfer of a bending quantum from one excited D2CO molecule to another is a crucial step. 
Collision-induced changes of rotational quantum number must also play an important role.

Michael R. Berman

C. Bradley Moore

The Small Molecule Limit of Infrared Multiphoton Dissociation: Collisional Effects in D2CO

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