Ionic Pathways following UV Photoexcitation of the (HI)2 van der Waals Dimer

Photodissociation of the (HI)2 van der Waals dimers at 248 nm and nearby wavelengths has been studied using time-of-flight mass spectrometry and velocity map imaging. I2+ product ions with a translational temperature of 130 K and “translationally hot” I+ ions with an average kinetic energy of Et = 1.24 ± 0.03 eV and angular anisotropy β = 1.92 ± 0.11 were detected as dimer-specific ionic photofragments. Velocity map images of the I2+ and I+ species were found to be qualitatively similar to those observed in the case of photoexcitation of the (CH3I)2 dimer (J. Chem. Phys.2005, 122, 204301). As in the case of the (CH3I)2 dimer, the absence of neutral I2-specific features in the ionic species images from (HI)2 allows us to eliminate neutral molecular I2 as a precursor of I+ and I2+. Similar to the case of (CH3I)2, we deduce that the observed I2+ ions are produced in their 2Π3/2,g ground electronic state as a result of photodissociation of the ionized dimer (HI)2+ + hν → I2+ + .... The formation of “translationally hot” I+ ions is attributed to photodissociation of nascent vibrationally excited I2+ with an average vibrational energy of 1.05 ± 0.10 eV. This vibrational excitation is explained by the nonequilibrium initial I−I distance in I2+ arising in photodissociation of (HI)2+ after prompt release of the light H atoms. On the basis of our ab initio calculated value for the I−I distance of (3.17 Å) in the (HI)2+ precursor dimer, the vibrational excitation of I2+ is expected to be 1.02 eV, which is in quantitative agreement with our experimentally deduced value. The interpretation of our results was supported by ab initio calculations of the structure and energy of neutral and ionized dimers of HI at the MP4(SDTQ)//MP2 level.

Vidma, Konstantin V. and Parker, David H. and Bogdanchikov, Georgii A. and Baklanov, Alexey V. and Kochubei, Sergei A.

The Journal of Physical Chemistry A 2010, 114, 9