Dynamics of I*(2P1/2) production from fluorinated alkyl iodides at 266, 280, and ∼305 nm

Abstract

In this paper, we present I*( 2 P 1/2 ) quantum yield, * from the gas phase photodissociation of a series of perfluoroalkyl iodides at three different wavelengths 266, 280, and ϳ305 nm. The iodine atoms in the ground I( 2 P 3/2 ) and spin-orbit excited I*( 2 P 1/2 ) states were monitored directly by a two photon laser induced fluorescence scheme. The I* quantum yields for the fluorinated alkyl iodides are found to be much higher than their corresponding alkyl iodide analogs over the entire A band. However, * remains more or less unchanged as a function of photolysis wavelength for the perfluoroalkyl iodides with the exception of CF 3 I in which it drops monotonically as a function of wavelength. Unlike in normal alkyl iodides, branching at the ␣ position does not affect the * in perfluoroalkyl iodides. The prediction of I* yield using the one dimensional Landau-Zener soft-radical-impulse model is opposite to what is seen from our measurements. In all the fluorinated alkyl iodides, the CF 3 in-plane rocking mode which can take the initially excited 3 Q 0 state to the 1 Q 1 state through the crossing region appears to be weakly coupled to the C-I stretch resulting in low yield of I atom in the dissociation process.

Photodissociation Study of Ethyl Bromide in the Ultraviolet Range by the Ion‐Velocity Imaging Technique

“…Gougousi and co-workers studied the photodissociation of CH 3 Br, in the wavelength range between 215 and 251 nm, [19] and found that b(Br*) appeared to be insensitive to the photolysis wavelength, while b(Br) displayed a non-monotonic decrease from positive to negative values as they move from the low-energy tail toward the peak of the absorption continuum. These results are very different from ours.…”

“…[11][12][13] Since the properties related to the transition are preserved due to its short dissociation time, the observed properties of the products (i. e., the vector quantities, internal energy distributions, and branching ratio of various product states) reflect the related potentialenergy surface (PES). Three repulsive electronic excited states have been predicted to contribute to the A-band absorption of alkyl halides, which have been identified as 3 Q 1 (2E), 3 Q 0 (2A 1 ), and 1 Q 1 (3E) by Mulliken and Teller. [14][15][16] As for alkyl bromides, the photodissociation via excitation to the A band produces ground-state bromine atoms, Br ( 2 P 3/2 ), and spin-orbit excited-state bromine atoms, Br ( 2 P 1/2 ), that we label as Br and Br*, respectively.…”

“…The major differences between them are the larger bond-dissociation energy of CÀBr compared to CÀI, and the larger spin-orbit coupling energy of the iodine atom compared to that of the bromine atom. The photodissociation of CH 3 Br was studied in the wavelength range of 215 to 251 nm by ion imaging, [19] and it was found that the transitions to the 3 Q 0 and 3 Q 1 states dominate the absorption cross-section and are an important contribution to the Br production from the curve crossing between the 3 Q 0 and 1 Q 1 states, whereas, in the 193 nm photolysis of CH 3 Br, [24] the curve crossing from the 3 Q 0 and 1 Q 1 surfaces has been barely observed with a rotatable quadrupole mass spectrometer. Ebenstein and co-workers [25] and Kim and co-workers [18] have investigated the photodissociation of CF 3 Br at 193 and 234 nm, respectively.…”

“…Many works have been completed on the photodissociation dynamics of a range of alkyl halides. [1][2][3][4][5][6][7][8][9][10] They undergo direct dissociation of the CÀX (X = Cl, Br, I) bond via a s* ! n transition of lone pair electrons of X at its first absorption A band.…”

Imaging the Translational Dynamics ofCF3in Liquid Helium Droplets

“…Excitation of CF 3 I through the A band in the gas phase leads to a prompt rupture of the C-I bond producing CF 3 and I fragments. Studies at various UV wavelengths show that most of the iodine fragments are formed in the spin-orbit excited 2 P 3=2 state [12] and that most of the fluoromethyl fragments are vibrationally excited [13][14][15]. From these data and the CF 3 I dissociation energy [16] the average total kinetic energy of the fragment pair at the dissociation wavelength of 266 nm can be estimated to be 1.25 eV.…”