Multiphonon vibrational excitation of polyatomic molecules in mixtures with foreign gases

Abstract

The role of collision effects in multiphoton vibrational excitation (MVE) of molecules is an active area of research at the present time [1--5]. Interest in this question stems from the little-studied characteristics of relaxation processes with high levels of vibrational excitation of polyatomic molecules, and also from possible practical applications in laser photochemistry. For polyatomie molecules, for which the region of discrete vibrational levels is narrower and the cross sections of multiphoton absorption differ less from the sections in the 0 ~ 1 transition than in the case of small molecules, the influence of collisions on MVE has not been studied as thoroughly. Of particular interest is the influence of collisions on MVE in two-component mixtures of absorbing molecules and foreign gases. When the molecules of such mixtures are excited by radiation from a CO s laser, a number of complex effects are observed; and no satisfactory and unambiguous interpretation has yet been given. Analysis of these effects may be greatly complicated by thermal processes, which often determine the effective time of photochemical reactions initiated by laser radiation in a confined volume [3--4]. In the work reported here, we investigated changes in absorbed energy, rates of relaxation processes, and intensities of induced absorption from high vibrational levels of the ground electronic state, as related to increases in the pressure of foreign gases in mixtures with polyatomic molecules of different degrees of complexity. The molecules that were investigated were aerolein (CaH40), acetophenone (CallaO), and diacetyl (C4HeOs); in the region of CO~. laser radiation, they have IR absorption bands with Vm~ x = 959,954, and 945 cm -1 and comparatively small absorption cross sections at a frequency of 944 cm -1 (P20) (5.5, 1.5, 1.15).10 -19 cm 2, respectively. The gas mixtures were admitted to a heated cuvette with a length of 40 cm. The absorbing molecules were diluted with a large quantity of He, N s, or Ar. In measuring the absorbed energy in the pressure interval from 0 to 2.0.106 Pa, we used a specially designed high-pressure steel cuvette. The IR radiation pulse had an energy up to 5 J, duration of peak 80 nsec at half-height, and a "tail" of about 1.5 #see. We used a collimated laser beam with a diameter (1.5 cm) that was smaller than the diameter of the cuvette (4 cm). The changes in cross section of the laser beam through the length of the cuvette were no greater than 30%. The absorbed energy was determined by means of standard calorimeters or pyroelectric sensors. The cuvette length and the vapor pressure were selected so that the vapor would absorb no more than 15% of the laser energy, so that we were able to consider the intensity of IR radiation as identical over the entire length of the cuvette, and the temperature distribution in the laser beam as uniform. The kinetics of change in populations of vibrational levels of the ground electronic state were monitored on the basis of electronic absorption. These observations were performed in the long-wave absorption bands of acrolein (320--390 nm), diacetyl (370--460 nm), and acetophenone (300--360 nm), and also in regions where there is no absorption under ordinary conditions. At various levels of CO s laser energy density Ecos, we determined the dependence of the absorbed energy (average per molecule) on the pressure of foreign gas Peg (Fig. 1). We found that, for these particular polyatomie molecules, the changes in cross section of MVE with increasing Peg are either very small or zero. Oscillograms of induced absorption on the long-wave shoulder of the electronic spectra, in the region where there is little or no original absorption, demonstrate that this absorption increases with increasing pressure of N~. or Ar for all of the molecules that were investigated. In Fig. 2 we show the influence of Pfg on the relative intensities I(t)/Io(t), where I(t) and I0(t) are the intensities at the maxima of the induced absorption pulses in the mixtures and in the pure vapor, respectively. An increase in electronic absorption when vapor with a pressure no higher than l0 s Pa is highly diluted with a foreign gas is observed only with pressures Pfg < 1.10 s Pa. Subsequently, the curves come out to saturation more