Methyl Iodide Photodissociation at 193 nm: The I(²P_1/2) Quantum Yield

Abstract

Methyl iodide photolysis at 193 nm has been studied through probing the I((2)P(1/2)-(2)P(3/2)) transition in the atomic iodine photofragment using diode laser spectroscopy. The I((2)P(1/2)) quantum yield has been determined through two different diode laser techniques and then compared. Frequency-modulated diode laser based absorption spectroscopy was used to extract nascent Doppler lineshapes from which an I((2)P(1/2)) quantum yield of unity is inferred. However when diode laser gain/absorption measurements were made, an I((2)P(1/2)) quantum yield of 0.68 ± 0.04 was found. The reason for this discrepancy is shown to lie in the diode laser gain/absorption method. Molecular iodine is found to be formed during the experiment via atomic iodine recombination and then in turn dissociates to produce both I((2)P(1/2)) and I((2)P(3/2)), thus distorting the returned quantum yield. This conclusion is supported both by the reduction of the I((2)P(1/2)) quantum yield with number of photolysis laser shots when measured using this technique and by the presence of fluoresence which is shown to have excited-state lifetimes and quenching rates that are consistent with those previously measured for the D and D' states of molecular iodine.

Time-resolved predissociation of the vibrationless level of the B state of CH3I

“…7,26,29,30 surface near its equilibrium geometry, and both diabatically correlate with the CH 3 +I* dissociation limit. 28,31 The predissociation is caused by the spin-orbit coupling.…”

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

“…Some controversy has existed in the literature over this issue, since values lower than unity had been found in diode laser gain/absorption measurements. 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.…”

“…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.…”

“…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.…”

“…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.…”