Raman spectroscopy and crystal-field split rotational states of photoproducts CO and H2 after dissociation of formaldehyde in solid argon

Abstract

Kiljunen, T. (2012). Raman spectroscopy and crystal-field split rotational states of photoproducts CO and H2 after dissociation of formaldehyde in solid argon. The Journal of Chemical Physics, 137 (16), 164310-11. doi:10.1063 Raman signal is monitored after 248 nm photodissociation of formaldehyde in solid Ar at temperatures of 9-30 K. Rotational transitions J = 2 ← 0 for para-H 2 fragments and J = 3 ← 1 for ortho-H 2 are observed as sharp peaks at 347.2 cm −1 and 578.3 cm −1 , respectively, which both are accompanied by a broader shoulder band that shows a split structure. The rovibrational spectrum of CO fragments has transitions at 2136.5 cm −1 , 2138.3 cm −1 , 2139.9 cm −1 , and 2149 cm −1 . To explain the observations, we performed adiabatic rotational potential calculations to simulate the Raman spectrum. The simulations indicate that the splitting of rotational transitions is a site effect, where H 2 molecules can reside in a substitution site, in addition to an interstitial site. In the former site, rotational motion is unperturbed by the electrostatic field of the host atoms, while the latter site splits the excited rotational manifolds, J = 2 and 3, into doublet and triplet structures, respectively. For CO, the spectrum can be ascribed to monomeric species in single-and double-substitution sites, to a dimeric species (CO) 2 , and to a CO-H 2 O complex. The simulations show that a nearest-neighbor molecular complex CO-H 2 is not responsible for any of the observed spectral fingerprints. The cause of the exit of the molecular hydrogen from the initial cage can be traced to high translational energy of the fragment after the photodissociation. After the matrix has reached a thermal equilibrium, a diffusion driven formation of the complex is possibly hindered by the high rotational zero-point energy developed upon complexation.

Radiation-induced transformations of methanol molecules in low-temperature solids: a matrix isolation study

“…Meanwhile, the dynamics of formaldehyde excited states produced in photolysis was studied by Lindgren. 79 According to this work, excess energy on the first excited singlet state S 1 is distributed among the vibrational modes of H 2 CO. In a solid environment, some of the vibrational energy is lost to the surrounding cage atoms in collisions and the dissociation outcome is determined by the details of collision dynamics.…”

“…The molecular products (H 2 and CO) are formed exclusively from S 0 , whereas the radical products (H and HCO ) can be formed from both T 1 and S 0 . 79 Thus, the conversion of excited methanol to CO and HCO may occur without intermediate stabilization of the formaldehyde molecule in the ground state. The efficiency of such processes should decrease with increasing efficiency of the matrix ''cooling'' (deactivation) of the excited formaldehyde molecules.…”

Excitation of rotons in parahydrogen crystals: The laser-induced-molecular-alignment mechanism

“…Molecular hydrogen is a suitable probe due to relatively weak interactions and its simple structure. 20,21 As a coherence-based control tool, efficient extension of the alignment mechanism to the condensed phase is posing challenges both experimentally and theoretically. [22][23][24][25] Therefore, parahydrogen (pH 2 ) as a minimally perturbing quantum condensed phase makes natural sense as the media to study.…”