A new look at the photodissociation of methyl iodide at 193 nm

Abstract

A new measurement of the photodissociation of CH3I at 193 nm is reported in which we use a combination of vacuum ultraviolet photoionization and velocity map ion imaging. The iodine photofragments are probed by single-photon ionization at photon energies above and below the photoionization threshold of I((2)P(3/2)). The relative I((2)P(3/2)) and I*((2)P(1/2)) photoionization cross sections are determined at these wavelengths by using the known branching fractions for the photodissociation at 266 nm. Velocity map ion images indicate that the branching fraction for I((2)P(3/2)) atoms is non-zero, and yield a value of 0.07 ± 0.01. Interestingly, the translational energy distribution extracted from the image shows that the translational energy of the I((2)P(3/2)) fragments is significantly smaller than that of the I*((2)P(1/2)) atoms. This observation indicates the internal rotational/vibrational energy of the CH3 co-fragment is very high in the I((2)P(3/2)) channel. The results can be interpreted in a manner consistent with the previous measurements, and provide a more complete picture of the dissociation dynamics of this prototypical molecule.

Structural dynamics effects on the electronic predissociation of alkyl iodides

“…Following excitation of CH 3 I at ~201 nm, an electronic predissociation through the coupling between the initially populated 3 R 1 and the repulsive 3 A 1 state leads to the mayor dissociation into CH 3 + I*. A small amount of iodine in its ground spin-orbit state in correlation with CH 3 remarkably ro-vibrationally excited was detected and attributed to a second curve crossing between 3 R 1 and the repulsive state 1 Q 1 9,11,12 . The corresponding quantum yield Φ * , defined as Φ * = [I * ]/([I * ] + [I]), was found to range between 0.99 in the 0 0 0 and 0.87 in the 3 0 1 vibronic band, i.e.…”

“…The corresponding quantum yield Φ * , defined as Φ * = [I * ]/([I * ] + [I]), was found to range between 0.99 in the 0 0 0 and 0.87 in the 3 0 1 vibronic band, i.e. excitation in the C-I stretching mode (v 3 ) 12 . Employing femtosecond VMI, a predissociation lifetime of 1.52 ± 0.10 ps was measured for the CH 3 + I* channel 5 .…”

Absorption spectra at the iodine 3d ionisation threshold following the CHxI+ (x = 0–3) cation sequence

“…Neutral iodomethane is intensively studied because it can easily be excited either by electrons, 3 by high-power lasers of 193 nm, [4][5][6][7][8] 266 nm, [9][10][11] or even longer wavelengths, [12][13][14][15][16][17][18] dissociating the molecule. 8,19,20 Photons of these different wavelengths, produced by the Sun, [21][22][23][24] can dissociate CH 3 I from natural sources [25][26][27] in Earth's atmosphere forming atomic iodine (ions) which act as catalysts for ozone depletion. [28][29][30] Unravelling the dissociation and excitation mechanisms of the iodomethane molecule is important to understand its role in the chemistry of Earth's atmosphere.…”

A velocity-map imaging study of methyl non-resonant multiphoton ionization from the photodissociation of CH 3 I in the A-band

“…In particular, the resonance enhanced multiphoton ionization (REMPI) technique provides highly resolved translational energy distributions (TEDs) of rovibrationally state-selected photofragments. By contrast, experimental studies on dissociation dynamics using non-resonant multiphoton ionization (NRMPI) detection are quite scarce and generally employ 800 nm MPI in femtosecond pump-probe schemes [5] or the so-called 'universal detection', which employs VUV radiation (at 118 nm or 121 nm) to ionize all the produced fragments with a single photon [6][7][8][9][10][11][12][13][14][15][16]. Femtosecond multiphoton excitation involves several (approx.…”

“…With ionization thresholds around 10 eV, alkyl radicals and halogen atoms, make the VUV detection particularly suitable for the study of the photodissociation of alkyl halides. In particular, Xu & Pratt [13,15] have used extensively the VUV detection technique in the photodissociation of methyl iodide in the second absorption band, namely the B-band, associated with the excitation to the ( 2 E 3/2 ) 6 s Rydberg state, and it has been recently employed to determine the I( 2 P 3/2 ) and I*( 2 P 1/2 ) branching ratio for different vibronic levels of the Rydberg state [15]. One-photon detection experiments are easily interpreted, but lack the capabilities that multiphoton resonant schemes provide.…”