Chirantha P. Rodrigo

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). II. Velocity map imaging studies

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ↔ CH(3)O (CD(2)OH ↔ CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).

Conformer-specific vibronic spectroscopy and vibronic coupling in a flexible bichromophore: Bis-(4-hydroxyphenyl)methane

The vibronic spectroscopy of jet-cooled bis-(4-hydroxyphenyl)methane has been explored using fluorescence excitation, dispersed fluorescence (DFL), UV-UV hole-burning, UV depletion, and fluorescence-dip infrared spectroscopies. Calculations predict the presence of three nearly isoenergetic conformers that differ in the orientations of the two OH groups in the para positions on the two aromatic rings (labeled uu, dd, and ud). In practice, two conformers (labeled A and B) are observed, with S(0)-S(1) origins at 35,184 and 35,209 cm(-1), respectively. The two conformers have nearly identical vibronic spectra and hydride stretch infrared spectra. The low-frequency vibronic structure is assigned to bands involving the phenyl torsions (T and T), ring-flapping (R and R), and butterfly (β) modes. Symmetry arguments lead to a tentative assignment of the two conformers as the C(2) symmetric uu and dd conformers. The S(0)-S(2) origins are assigned to bands located 132 cm(-1) above the S(0)-S(1) origins of both conformers. DFL spectra from the S(2) origin of the two conformers display extensive evidence for vibronic coupling between the two close-lying electronic states. Near-resonant coupling from the S(2) origin occurs dominantly to S(1) R(1) and S(1) R(1)β(1) levels, which are located -15 and +31 cm(-1) from it. Unusual vibronic activity in the ring-breathing (ν(1)) and ring-deformation (ν(6a)) modes is also attributed to vibronic coupling involving these Franck-Condon active modes. A multimode vibronic coupling model is developed based on earlier theoretical descriptions of molecular dimers [Fulton and Gouterman, J. Chem. Phys. 35, 1059 (1961)] and applied here to flexible bichromophores. The model is able to account for the ring-mode activity under conditions in which the S(2) origin is strongly mixed (60%/40%) with S(1) 6a(1) and 1(1) levels. The direct extension of this model to the T/T and R/R inter-ring mode pairs is only partially successful and required some modification to lower the efficiency of the S(1)/S(2) mixing compared to the ring modes.

Accessing Multiple Conical Intersections in the 3s and 3px Photodissociation of the Hydroxymethyl Radical

The photodissociation dynamics of the hydroxymethyl radical (CH2OH, CH2OD, and CD2OD) following excitation to the 3s and 3p(x) Rydberg states is studied using time-sliced velocity map imaging of hydrogen photofragments. Dissociation takes place on the ground potential energy surface reached via conical intersections from the excited states, and formaldehyde and hydrxymethylene are identified as reaction products. The major product, formaldehyde, has a bimodal internal energy distribution. The largest fraction has high kinetic energy (KE), modest rotational excitation, and vibrational excitation mainly in the CO stretch and the CH(D)2 deformations modes (scissors, wag, and rock). The minor fraction has lower KEs and a higher rovibrational excitation that is unresolved. A bimodal internal energy distribution in the formaldehyde fragment has been predicted by Yarkony [J. Chem. Phys. 2005, 122, 084316] for a conical intersection along the O-H bond coordinate. The hydroxymethylene product state distributions depend strongly on the nature of the excited state. In dissociation via the 3s state, the hydroxymethylene products have broad rovibrational state distributions and are produced at low yield. As suggested by Yarkony, they may be produced in the same dissociation events that give rise to low KE formaldehyde. In these events, the bound region of the PES is sampled following the conical intersection along O-H(D). The hydroxymethylene yield is low near its threshold and increases slowly with excitation energy to the 3s state, but its internal energy distribution remains broad and the contributions of the cis- and trans-isomers cannot be resolved. The mechanism changes markedly when exciting to the 3p(x) state. The hydroxymethylene products have less rotational excitation and show separate contributions of cis- and trans-isomers. The trans-isomer is found to be a minor product relative to the higher-energy cis-isomer, as predicted by Yarkony for conical intersections along the C-H coordinate. It appears that the efficiency of dissociation via conical intersections along the O-H and C-H coordinates depends on the initial excited state. While the O-H conical intersection seam (vertical cone) provides an efficient route to the ground state following excitation via the 3s or the 3p(x) Rydberg states, conical intersections along the C-H bond coordinate (tilted cone) are sampled more efficiently via 3p(x) excitation and proceed through different dynamics. The energy separations between formaldehyde and hydroxymethylene and between the cis- and trans-isomers of hydroxymethylene are determined experimentally for all the investigated isotopologs and are in good agreement with theory.

Rotationally resolved C2 symmetric conformers of bis-(4-hydroxyphenyl)methane: prototypical examples of excitonic coupling in the S1 and S2 electronic states.

Rotationally resolved microwave and ultraviolet spectra of jet-cooled bis-(4-hydroxyphenyl)methane (b4HPM) have been obtained using Fourier-transform microwave and UV laser/molecular beam spectrometers. A recent vibronic level study of b4HPM [Rodrigo, C. P.; Müller, C. W.; Pillsbury, N. R.; James, W. H., III; Plusquellic, D. F.; Zwier, T. S. J. Chem. Phys. 2011, 134, 164312] has assigned two conformers distinguished by the orientation of the in-plane OH groups and has identified two excitonic origins in each conformer. In the present study, the rotationally resolved bands of all four states have been well-fit to asymmetric rotor Hamiltonians. For the lower exciton (S(1)) levels, the transition dipole moment (TDM) orientations are perpendicular to the C(2) symmetry axes and consist of 41(2):59(2) and 34(2):66(2)% a:c hybrid-type character. The S(1) levels are therefore delocalized states of B symmetry and represent the antisymmetric combinations of the zero-order locally excited states of the p-cresol-like chromophores. The TDM polarizations of bands located at ≈132 cm(-1) above the S(1) origins are exclusively b-type and identify them as the upper exciton S(2) origin levels of A symmetry. The TDM orientations and the relative band strengths from the vibronic study have been analyzed within a dipole-dipole coupling model in terms of the localized TDM orientations, μ(loc), on the two chromophores. The out-of-the-ring plane angles of μ(loc) are both near 20° and are similar to results for diphenylmethane [Stearns, J. A.; Pillsbury, N. R.; Douglass, K. O.; Müller, C. W.; Zwier, T. S.; Plusquellic, D. F. J. Chem. Phys. 2008, 129, 224305]. The in-plane angles are, however, rotated by 14 and 18° relative to DPM and, in part, explain the smaller than expected exciton splittings of these two conformers.

Imaging Studies of Excited and Dissociative States of Hydroxymethylene Produced in the Photodissociation of the Hydroxymethyl Radical

Rotational, vibrational, and electronic states of formaldehyde and cis-hydroxymethylene products generated in the photodissociation of the hydroxymethyl radical are investigated by sliced velocity map imaging (SVMI) following excitation of the radical to its 3px and 3pz Rydberg states. SVMI of H and D photofragments is essential in these studies because it allows zooming in on low-velocity regions of the images where small threshold signals can be identified. With CH2OD precursors, formaldehyde and hydroxymethylene products are examined separately by monitoring D and H, respectively. Whereas the main dissociation channels lead to formaldehyde and cis-hydroxymethylene in their ground electronic states, at higher excitation energies the kinetic energy distributions (KEDs) of H and D photofragments exhibit additional small peaks, which are assigned as triplet states of formaldehyde and hydroxymethylene. Results obtained with deuterated isotopologs of CH2OH demonstrate that the yield of the triplet state of formaldehyde decreases upon increasing deuteration, suggesting that the conical intersection seams that govern the dynamics depend on the degree of deuteration. The rotational excitation of cis-hydroxymethylene depends on the excited Rydberg state of CH2OD and is lower in dissociation via the 3pz state than via the lower lying 3px and 3s states. Vibrational excitation of cis-HCOD, which spans the entire allowed internal energy range, consists mostly of the CO-stretch and in-plane bend modes. When the internal energy of cis-HCOD exceeds the dissociation threshold to D + HCO, slow D and H photofragments deriving from secondary dissociation are observed. The yields of these H and D fragments are comparable, and we propose that they are generated via prior isomerization of cis-HCOD to HDCO.