Ab initioconfiguration interaction study of theB- andC-band photodissociation of methyl iodide

Abstract

Multireference spin-orbit configuration interaction calculations have been carried out for the valence and low-lying Rydberg states of CH(3)I. Potential energy surfaces along the C-I dissociation coordinate (minimal energy paths with respect to the umbrella angle) have been obtained as well as transition moments for excitation of the Rydberg states. It is shown that the B and C absorption bands of CH(3)I are dominated by the perpendicular (3)R(1),(1)R (E)←X̃ A(1) transitions, while the (3)R(2)(E), (3)R(0(+) )(A(1))←X̃ A(1) transitions are very weak. It is demonstrated that the bound Rydberg states of the B and C bands are predissociated due to the interaction with the repulsive E and A(2) components of the (3)A(1) state, with the (3)A(1)(E) state being the main decay channel. It is predicted that the only possibility to obtain the I((2)P(3/2)) ground state atoms from the CH(3)I photodissociation in the B band is by interaction of the (3)R(1)(E) state with the repulsive (1)Q(E) valence state at excitation energies above 55,000 cm(-1). The calculated ab initio data are used to analyze the influence of the Rydberg state vibrational excitation on the decay process. It is shown that, in contrast to intuition, excitation of the ν(3) C-I stretching mode supresses the predissociation, whereas the ν(6) rocking vibration enhances the predissociation rate.

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

“…19,33 However, we believe that consensus has been reached in this sense that the diode laser measurements were distorted due to the presence of molecular iodine. 19 All the other previous measurements of this quantity 15,17,18,24,32 have yielded unity, except the recent work by González et al, 34 which will be discussed below. In his very recent theoretical work, Alekseyev and co-workers 18 conclude that the only possibility to obtain I( 2 P 3/2 ) ground state atoms after B-band excitation is by interaction with the repulsive A-band 1 Q 1 (E) state (plotted in Fig.…”

“…It would seem in principle counterintuitive that excitation precisely in ν 3 (the mode of elongation of the C-I bond) corresponds to a slower dissociation. However, as pointed out by Alekseyev et al, 18 ab initio calculations show that the 3 A 1 repulsive state crosses the B-band very close to the equilibrium C-I distance of the latter. In this case, the ground state wave function would have a larger overlap with the degenerate continuum state than the ν 3 = 1 level, with a node in the C-I dimension near the position of the curve crossing, which would imply a lower coupling and therefore a slower predissociation.…”

“…Figure 14 shows the detail of the potential energy curves that intervene in the excitation/deactivation upon irradiation in this part of the spectrum, according to the ab initio calculations of Alekseyev and co-workers, 18 and the dashed lines show the Franck-Condon region. As can be inferred from the graph, the extreme sensitivity to the vibrational excitation is related to the details of the crossings between the potential energy surfaces and the spatial distributions of the wave functions at each vibrational level.…”

“…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. The angular nature of the process is also explored through the use of linearly polarized laser fields and a position-sensitive particle detector.…”

“…dissociation process. Also very recently, Alekseyev et al 18 published new ab initio potential energy curves and transition moments in one dimension (1D) (minimal energy paths with respect to the umbrella angle) on this system. They found that the B-band is dominated by the perpendicular 3 R 1 ← XA 1 transition and that the main predissociation channel occurs via interaction with the repulsive 4E( 3 A 1 ) state, which asymptotically yields spin-orbit excited iodine atoms: I*( 2 P 1/2 ) + CH 3 .…”