Infrared rovibrational spectroscopy of OH–C2H2 in 4He nanodroplets: Parity splitting due to partially quenched electronic angular momentum

Abstract

The T-shaped OH-C2H2 complex is formed in helium droplets via the sequential pick-up and solvation of the monomer fragments. Rovibrational spectra of the a-type OH stretch and b-type antisymmetric CH stretch vibrations contain resolved parity splitting that reveals the extent to which electronic angular momentum of the OH moiety is quenched upon complex formation. The energy difference between the spin-orbit coupled (2)B1 (A″) and (2)B2 (A') electronic states is determined spectroscopically to be 216 cm(-1) in helium droplets, which is 13 cm(-1) larger than in the gas phase [Marshall et al., J. Chem. Phys. 121, 5845 (2004)]. The effect of the helium is rationalized as a difference in the solvation free energies of the two electronic states. This interpretation is motivated by the separation between the Q(3/2) and R(3/2) transitions in the infrared spectrum of the helium-solvated (2)Π3/2 OH radical. Despite the expectation of a reduced rotational constant, the observed Q(3/2) to R(3/2) splitting is larger than in the gas phase by ≈0.3 cm(-1). This observation can be accounted for quantitatively by assuming the energetic separation between (2)Π3/2 and (2)Π1/2 manifolds is increased by ≈40 cm(-1) upon helium solvation.

On the Stark effect in open shell complexes exhibiting partially quenched electronic angular momentum: Infrared laser Stark spectroscopy of OH–C2H2, OH–C2H4, and OH–H2O

“…The parameter q in the quenching Hamiltonian therefore allows for a smooth transition in the model between the weak (Hund's case (a) coupling) and strong (Hund's case (b) coupling) interaction limits. For all three systems considered here, the two lowest energy electronic states ( 2 B 1 and 2 B 2 ) at equilibrium correlate, respectively, to the 2 P 3/2 and 2 P 1/2 levels of the OH fragment as the intermolecular separation increases [23].…”

“…The coordinate system employed here is the same as that described previously for the T-shaped OH-C 2 H 2 complex [20,23], which is shown in Fig. 1.…”

“…The terms associated with the zero-field problem have been discussed in detail previously [20,21,23,25]. Only the lowest energy, doubly degenerate g ¼ 1 state is considered, and only the …”

“…The matrix elements of b H rot and b H CD used here are the same as those discussed previously [20,23]. The rotational and centrifugal distortion constants determined from simulations of the zero-field spectra are retained in simulations of Stark spectra without modification.…”

“…However, as the strength of the intermolecular interaction increases, the electronic angular momentum becomes partially quenched due to the incipient barrier to free orbital motion of the electron about the OH axis. A zero-field model Hamiltonian and simulated spectra for these systems have been reported by Marshall and Lester [20,21], and their model has been applied successfully to interpret the rovibrational infrared (IR) spectra of the T-shaped OH-C 2 H 2 complex [22,23] and the microwave spectra of the OH-H 2 O complex [24][25][26]. Using a model Hamiltonian adapted from this previous zero-field work, simulations are shown to be in excellent agreement with the Stark spectra of the OH-C 2 H 2 , OH-C 2 H 4 , and OH-H 2 O complexes formed in helium nanodroplets.…”