A femtosecond velocity map imaging study on B-band predissociation in CH3I. I. The band origin

Abstract

A femtosecond pump-probe experiment, coupled with velocity map ion imaging, is reported on the second absorption band (B-band) of CH(3)I. The measurements provide a detailed picture of real-time B-band predissociation in the band origin at 201.2 nm. Several new data are reported. (i) A value of 1.5+/-0.1 ps has been obtained for the lifetime of the excited state, consistent within errors with the only other direct measurement of this quantity [A. P. Baronavski and J. C. Owrutsky, J. Chem. Phys. 108, 3445 (1998)]. (ii) It has been possible to measure the angular character of the transition directly through the observation of fragments appearing early with respect to both predissociation lifetime and molecular rotation. (iii) Vibrational activity in CH(3) has been found, both in the umbrella (nu(2)) and the symmetric stretch (nu(1)) modes, with estimates of relative populations. All these findings constitute a challenge and a test for much-wanted high level ab initio and dynamics calculations in this energy region.

A femtosecond velocity map imaging study on B-band predissociation in CH3I. II. The $2_0^1$21 and $3_0^1$31 vibronic levels

“…23 The time-resolved work of Baronavski and Owrustky 10 provided the first direct measurements of the lifetimes of many vibrational levels of the B-band and concluded that the lifetimes could be as extreme as 200 fs (for the ν 6 = 2 level) and 4100 fs (for the ν 3 = 1 level). Even though the lifetime found for the vibrationless level has been measured recently [15][16][17] and found in agreement with Baronavski and Owrustky's value, no other measurement of the lifetimes for the rest of the vibronic levels has been reported. We partially cover this matter in this work with measurements of the lifetimes of the ν 2 = 1 and ν 3 = 1 levels through a variety of time-resolved techniques.…”

“…Even stronger uncertainty remained as to the nature of the products of the predissociation process as a function of the vibronic level initially excited, since very scarce information is available on the matter, and it concerns only either the band origin [15][16][17] or the 2 2 0 band, since this last one can be accessed through the well known ArF laser transition at 193.3 nm. 19,24 Theoretical information is also scarce, the most valuable source at present being the very recent 1D work by Alekseyev et al 18 The experimental results that will be shown in this paper tackle the two main issues of importance: the degree and nature of vibrational excitation in the methyl fragment resulting from the predissociation process, and whether the branching ratio between the spin-orbit excited and the ground-state iodine channels, defined as *=[I*]/([I] + [I*]), is unity or less than unity.…”

“…Upon such excitation the molecule predissociates due to crossings with repulsive states, and therefore the spectrum shows clear vibrational structure [10][11][12][13][14] with peaks that are lifetime broadened. This system has historically received much less attention than the A-band, but a number of works have appeared in the literature recently [15][16][17] mainly exploring the details of the fast prea) Electronic mail: banares@quim.ucm.es. dissociation process.…”

“…This finding is consistent with the I*( 2 P 1/2 ) yield experimentally found to be unity in recent studies. 15,16,19 One of the most interesting aspects of the B-band system is the fact that the details of the curve crossings between the bound Rydberg states and the repulsive states should have profound influences on the outcome of the process, so that significant differences in the dynamics should be found between different vibrational levels of the optically active 3 R 1 state. Early Raman scattering experiments 20,21 seemed to indicate the opposite, since the same predissociation rates were extracted for all vibronic excitations, although in a later contribution 22 the authors did report a longer lifetime for excitation in the ν 3 mode and a significantly shorter lifetime for excitation in the ν 1 level.…”

Multiple ionization of CH3I clusters by nanosecond laser: Electron energy distribution and formation mechanism of multiply charged ions

“…CH 3 I, a symmetric top molecule, has been used extensively to study the ionization and dissociation dynamic process [1][2][3]. Recently, the interaction of nanosecond laser with CH 3 I cluster was investigated widely, where multiply charged ions with high energy were observed experimentally [4][5][6][7].…”