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

Abstract

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band (B-band) of CH 3 I, including vibrational excitation in the ν 2 and ν 3 modes of the bound 3 R 1 (E) Rydberg state. Specific predissociation lifetimes have been determined for the 2 The result, previously reported in the literature, where vibrational excitation to the C-I stretching mode (ν 3 ) of the CH 3 I 3 R 1 (E) Rydberg state yields a predissociation lifetime about four times slower than that corresponding to the vibrationless state, whereas predissociation is twice faster if the vibrational excitation is to the umbrella mode (ν 2 ), is confirmed in the present experiments. In addition to the specific vibrational state lifetimes, which were found to be 0.85 ± 0.04 ps and 4.34 ± 0.13 ps for the 2 1 0 and 3 1 0 vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH 3 fragment are presented. Additional striking results found in the present work are the evidence of ground state I( 2 P 3/2 ) fragment production when excitation is produced specifically to the 3 1 0 vibronic level, which is attributed to predissociation via the A-band 1 Q 1 potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A-band 3 A 1 (4E) state, after direct absorption to this state, competing with absorption to the 3 1 0 vibronic level of the 3 R 1 (E) Rydberg state of the B-band.

Dynamic Stark shift of the3R1Rydberg state of CH3I

“…An example of images obtained in these conditions is presented in the inset of figure 1. The three almost isotropic rings -the inner one is only barely seen-correspond to CH 3 fragments in different vibrational states (ν 1 = 0, 1, 2; ν 2 = 0) [5].…”

“…All beams are focused with a 25 cm focal length lens onto a pulsed molecular beam. The ions generated in the interaction region are mass separated in a time-of-flight mass spectrometer and detected by velocity map imaging (VMI) [5]. The methodology employed to measure the Stark shift is based on retuning the pump laser wavelength to recover the resonance between the vibrationless ground and 3 R 1 states.…”

“…The experimental setup has been described previously [5]. Three laser pulses are required for this experiment.…”

Structural dynamics effects on the ultrafast chemical bond cleavage of a photodissociation reaction

“…Kinetic energy-radius calibration is done by measuring resonantly ionized CH 3 (n = 0) fragments at 333.45 nm after photodissociation of CH 3 I at 268 nm, taking advantage of the well known kinetic energy release (KER) for both I*( 2 P 1/2 ) and I( 2 P 3/2 ) yielding channels. Ion images are Abel inverted using the polar basis set expansion (pBASEX) 12 method and a Levenberg-Marquardt squared residuals minimization algorithm 13 is used for fits to the data. The instrument temporal response time, considered as the temporal cross-correlation of the pump and probe pulses, was measured by 1+1 0 multiphoton ionization of Xe, obtaining a value of 150 fs.…”

Multidimensional Analysis of Time-Resolved Charged Particle Imaging Experiments

“…It is important to note that the multidimensional nature of the fit allows the discrimination of the different contributions to the images, in a manner that a reduced-dimensionality analysis cannot achieve. This method has revealed its value in a number of contributions where the need for discrimination of overlapping contributions in charged particle images was crucial [13][14][15][16][17][18]. The method is analogous to the global 2D fit approach used in Stolow's group [19][20][21][22] but can easily be extended to extra dimensions such as the anisotropy, the temperature, the intensity dependence, measurements in coincidence, etc.…”