Small (HCl) m (H2O) n clusters have been assembled in He droplets, and their spectra in the HCl stretch range (2500−3000 cm−1) have been obtained. In a recent He droplet study, a band at 2670 cm−1 was assigned to the dissociated H3O+(H2O)3Cl− ion pair. In this work, we have revised the assignment of this band to a cyclic hydrogen-bonded form of the (HCl)2(H2O)2 cluster based on careful measurements of the pickup pressure dependence as well as the transition moment angles associated with the HCl stretch vibrations. A number of vibrational bands due to small mixed clusters have also been observed. As the number of the captured water molecules increases, a broad feature appears that spans the 2550−2800 cm−1 range. The possible origin of this spectral broadening in large (HCl) m (H2O) n clusters is discussed.

Infrared spectra in the HCl stretch region (2600-2900 cm(-1)) are presented for small, mixed (HCl)(m)(H(2)O)(n) clusters solvated in helium nanodroplets. Sharp bands associated with the Cl-H...Cl stretch vibrations in m:n = 1:1, 2:1, 2:2, and 3:1 clusters are superimposed on a broad background that increases in intensity as larger clusters are grown within the droplets. The broad background is determined to be partially due to mixed clusters with m > 3 and n > 2. The sharp bands are assigned to specific cluster compositions, m:n, via pick-up pressure dependence studies and optically selected mass spectrometry. Orientation of the clusters is achieved with the application of a large electric field to the laser/droplet beam interaction region. The intensity of each band is measured as a function of the applied field strength. Simulations of this electric field dependence based on ab initio calculations of permanent dipole moments and vibrational transition moment angles leads to definitive structural assignments for each sharp band. The 2:1 complex is cyclic, and a band associated with the 2:2 cluster is determined to arise from the nonalternating cyclic structure.

We report the automation of a continuous-wave, singly resonant, optical parametric oscillator (Lockheed-Martin Aculight ARGOS 2400-SF-15). This commercially available optical parametric oscillator (OPO) is capable of producing >1 W of continuously tunable idler output between 2.2 and 4.6 μm. An algorithm based on the feedback from a high accuracy wavemeter is implemented to synchronize three separate OPO tuning elements; the translation of a fan-out type periodically poled lithium niobate crystal, the rotation of an intracavity etalon, and the continuous tuning of the pump and idler wavelengths via piezoelectric strain of the tunable fiber pump laser. This allows for several hundred wavenumbers of efficient, automatic, continuous tuning of the idler wave. Continuous feedback from the wavemeter limits the absolute frequency accuracy to ±20 MHz. The broad, automatic tuning of the OPO is demonstrated via its implementation as a probe laser for the infrared action spectroscopy of methanol solvated in helium nanodroplets. LabVIEW virtual instruments for the automation of this OPO laser system are reported, along with detailed schematics of the associated hardware developed at the University of Georgia.

A combination of liquid He droplet experiments and multireference electronic structure calculations is used to probe the potential energy surface for the reaction between the propargyl radical and O2. Infrared laser spectroscopy is used to probe the outcome of the low temperature, liquid He-mediated reaction. Bands in the spectrum are assigned to the acetylenic CH stretch (ν1), the symmetric CH2 stretch (ν2), and the antisymmetric CH2 stretch (ν13) of the trans-acetylenic propargyl peroxy radical ((•)OO-CH2-C≡CH). The observed band origins are in excellent agreement with previously reported anharmonic frequency computations for this species [Jochnowitz, E. B.; Zhang, X.; Nimlos, M. R.; Flowers, B. A.; Stanton, J. F.; Ellison, G. B. J. Phys. Chem. A 2010, 114, 1498]. The Stark spectrum of the ν1 band provides further evidence that the reaction leads only to the trans-acetylenic species. There are no other bands in the CH2 stretching region that can be attributed to any of the other three propargyl peroxy isomers/conformers that are predicted to be minimum energy structures (gauche-acetylenic, cis-allenic, and trans-allenic). There is also no evidence for the kinetic stabilization of a van der Waals complex between propargyl and O2. A combination of multireference and coupled-cluster electronic structure calculations is used to probe the potential energy surface in the neighborhood of the transition state connecting reactants with the acetylenic adduct. The multireference based evaluation of the doublet-quartet splitting added to the coupled-cluster calculated quartet state energies yields what are likely the most accurate predictions for the doublet potential curve. This calculation suggests that there is no saddle point for the addition process, in agreement with the experimental observations. Other calculations suggest the possible presence of a small submerged barrier.

We report the ro-vibrational spectrum of the ν3(e') band of the methyl radical (CH3) solvated in superfluid (4)He nanodroplets. Five allowed transitions produce population in the N(K) = 0(0), 1(1), 1(0), 2(2) and 2(0) rotational levels. The observed transitions exhibit variable Lorentzian line shapes, consistent with state specific homogeneous broadening effects. Population relaxation of the 0(0) and 1(1) levels is only allowed through vibrationally inelastic decay channels, and the (P)P1(1) and (R)R0(0) transitions accessing these levels have 4.12(1) and 4.66(1) GHz full-width at half-maximum line widths, respectively. The line widths of the (P)R1(1) and (R)R1(1) transitions are comparatively broader (8.6(1) and 57.0(6) GHz, respectively), consistent with rotational relaxation of the 2(0) and 2(2) levels within the vibrationally excited manifold. The nuclear spin symmetry allowed rotational relaxation channel for the excited 1(0) level has an energy difference similar to those associated with the 2(0) and 2(2) levels. However, the (P)Q1(1) transition that accesses the 1(0) level is 2.3 and 15.1 times narrower than the (P)R1(1) and (R)R1(1) lines, respectively. The relative line widths of these transitions are rationalized in terms of the anisotropy in the He-CH3 potential energy surface, which couples the molecule rotation to the collective modes of the droplet.