Photoionization of CH₃I mediated by the C state in the visible and ultraviolet regions

Abstract

Three/two-photon resonant multiphoton ionization (MPI) of the CH3I monomer has been studied in the gas phase at 532 and 355 nm using time-of-flight mass spectrometry. Under low laser intensity (approximately 10(9) W/cm2) the mass spectra showed peaks at m/z 15, 127 and 142, corresponding to [CH3]+, [I]+ and [CH3I]+ species, at both these wavelengths. The laser power dependence for [CH3I]+, [I]+ and [CH3]+ ions showed a three-photon dependence at 532 nm. For the same three ions, photoionization studies at 355 nm gave a power dependence of 2. Both these results suggest that a vibronic energy level at approximately 7 eV, lying in the Rydberg C state, acts as a resonant intermediate level in ionization of CH3I. In the case of 355 nm, with increasing intensity additional peaks at m/z 139 and 141 were observed which could be assigned to [CI]+ and [CH2I]+ fragments. In contrast, for high intensity radiation at 532 nm ( approximately 2 x 10(10) W/cm2), only the [CI]+ fragment was observed. At these wavelengths, fragment ions observed in mass spectra mainly arise from photodissociation of the parent ion. Experiments at another wavelength in the visible region (564.2 nm) confirmed the results obtained at 532 nm. In order to assess the role of the A state in these MPI experiments, additional experiments were performed at 266 and 282.1 nm, which access the A state directly via a one-photon transition, and showed absence of a surviving precursor ion. Reaction energies for various possible dissociation channels of CH3I/[CH3I]+/[CH2I]+ were calculated theoretically at the MP2 level using the GAMESS electronic structure program.

Dissociative ionization and Coulomb explosion of CH3I in intense femto second laser fields

“…It requires the absorption of seven 800 nm fs photons to emit one electron of the outer shell nonbonding orbital. The resulting intermediate CH 3 [47,49]. The large Franck Condon factor associated with these nearly similar rotational constants strongly enhances the multi photon absorption rate for this particular transition.…”

“…All observed ion images, shown in the left column of Figure 2 are almost identical to the four-fold symmetrical ones in the central column. The panels in the right column of Figure 2 display the Abel inverted cylindrical symmetric recoil velocity distribution of the I q+ fragments with respect to the polarization vector, as obtained with the "Davis62" computer code [47].…”

“…The large Franck Condon factor associated with these nearly similar rotational constants strongly enhances the multi photon absorption rate for this particular transition. Moreover, the CH 3 I + [ 2 E 3/2 (X)] ion has to absorb at least three additional 800 nm photons to rise from the 22 018 cm −1 deep PES well above the dissociation limit to the CH 3 (X 2 A ) + I + ( 3 P j ) fragment channel of CH 3 I + itsà state [47,48]. This dissociation process disposes the CH 3 radical fragment in the vibronic ground state [50] and I + ( 3 P j ) in the j = 2 electronic ground state or in fragment ion components born after laser irradiation at an intensity of 3.7 × 10 14 W/cm 2 .…”

Interaction of nanosecond laser pulse with tetramethyl silane (Si(CH3)4) clusters: Generation of multiply charged silicon and carbon ions

“…[10][11][12][13][14][15][16][17] Additionally, the above mechanism also provides logical explanation for the systematic evolution of charge state observed with increasing argon stagnation pressure as shown in Figure 2. Increasing the argon stagnation pressure results in an increase in the three body collision rate and hence the average size of the cluster.…”

“…Details of the experimental setup have been described in our earlier publications [10][11][12][13][14][15][16][17] and only a brief account is given here. Neutral clusters were generated via supersonic expansion of TMS vapors seeded in helium/argon at a stagnation pressure of 1-6 bar, at room-temperature.…”