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We have investigated the solvent response after photoexcitation using Optical Pump THz Probe (OPTP) spectroscopy with sub-ps time resolution in combination with quantum-chemical modeling. Excited State Proton Transfer (ESPT) is found to be facilitated by excitation of an intermolecular stretch (Hpyranine-Owater) triggered by an instantaneous (< 100 fs) redistribution of charge density. Initiation of proton transfer is observed on a time scale of 0.4 ps, i.e. on a much shorter time scale than anticipated before. Our results demonstrate the exceptional sensitivity of OPTP, which enables us to follow the proton motion into the hydrogen bond network of water exploring charge transport in transient solvent configurations in real time.

The authors have recorded the ν3 infrared spectrum of methane in helium nanodroplets using our cw infrared optical parametric oscillator. In a previous paper, Nauta and Miller [Chem. Phys. Lett. 350, 225 (2001)] reported the observation of the monomer rovibrational transitions of methane in helium nanodroplets. Here, they report the observation of additional absorption bands in the frequency range between 2990 and 3070 cm-1 blueshifted compared to the monomer transitions. They attribute these absorption features to phonon wings of individual rovibrational transitions, i.e., the simultaneous excitation of collective excitation modes of the quantum fluid and the rovibrational excitation of the methane monomer in the helium nanodroplet. [ABSTRACT FROM AUTHOR]

We report the first fully analyzed high resolution spectrum of a carboxylic acid dimer in the gas phase. High resolution spectra in the region of the C-O stretch 1241.7-1250.7 cm[SUP-1] have been recorded for (DCOOH)[SUB2]. The data could be fit within experimental uncertainty to a rigid rotor Watson S reduced Hamiltonian. The vibrational frequency of the C-O stretch in (DCOOH)[SUB2] is determined to be 1244.8461 (2) cm[SUP-1]. Our spectra are the first direct experimental evidence for proton transfer tunneling in formic acid dimer (FAD), with FAD serving as a prototype for double hydrogen bonded organic complexes. Previous theoretical studies predicted proton transfer times covering a range of several orders of magnitude. Our measurements on (DCOOH)[SUB2] established a proton transfer time of 5.8 ns [tunneling splitting of 0.00286(25) cm[SUB-1]]. The proton transfer was found to accelerate upon vibrational excitation of the skeleton motion (the vibrational C-O mode), corresponding to a tunneling splitting of 0.00999(21) cm[SUP-1]. For this state the proton transfer time decreased to 1.7 ns, which corresponds to a vibrationally enhanced proton transfer mechanism. The moments of inertia are in agreement with a C[SUB2h] structure which indicates a high tunneling barrier. The observed spin statistical weights follow a description of formic acid dimer in the molecular symmetry group G[SUB8], which is isomorphic to the point group D[SUB2h]. Our results suggest an out-of-plane contribution to the proton transfer mechanism for the otherwise planar dimer. [ABSTRACT FROM AUTHOR]

We have performed microwave–infrared double resonance measurements on OCS in pure [sup 4]He and mixed [sup 4]He/[sup 3]He clusters. In contrast to our previous study on OCS in pure [sup 4]He cluster, where an asymmetric inhomogeneous linewidth increasing with rotational quantum number j was observed [S. Grebenev, M. Havenith, F. Madeja, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys. 113, 9060 (2000)], we find the lines in the mixed helium cluster to be much narrower, showing a symmetric, probably homogeneous line width. The spectra indicate a significant difference in the rotational relaxation of OCS in mixed clusters as compared to pure [sup 4]He cluster. We have applied the model of Lehmann [K. K. Lehmann, Mol. Phys. 97, 645 (1999)], describing the interaction of a molecule with the helium cluster, and modified it in order to describe particle in a box states of OCS in mixed helium clusters. The calculations indicate while in the mixed cluster the OCS is confined to the inner [sup 4]He core, the energy of the particle in the box states is increased as compared to the pure [sup 4]He cluster. Relaxation mechanisms via these particle in the box states and surface excitations (ripplons) of the cluster are discussed in the text. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A direct systematic comparison between the terahertz spectra and the dynamics of solvated mono- and disaccharides is reported. The obtained results show that the solute-induced change in hydration dynamics is directly related to the number of hydrogen bonds formed between water molecules and the sugar.

The infrared spectra of HCl, [(HCl).sub.2], and [H.sub.2]O-HCl in liquid helium nanodroplets are analyzed. The HCl-[H.sub.2]O complex has very large red shift and broadening of the transition, which is due to the interaction of the helium environment with low-lying vibrations of the complex.

Over 150 lines in six tunneling subbands of an intermolecular vibration located near 25 cm-1 have been measured with partial hyperfine resolution and assigned to (NH3)2. The transitions sample all three types of tunneling states (A, G, E) and are consistent with the following assumptions: (1) G36 is the appropriate molecular symmetry group; (2) the equilibrium structure contains a plane of symmetry; (3) interchange tunneling of inequivalent monomers occurs via a trans path; (4) the 2C3+I limit of hydrogen exchange tunneling is appropriate; (5) tunneling and rotational motions are separable. A qualitative vibration–rotation tunneling energy level diagram is presented. Strong perturbations are observed among the states of E symmetry. This work supports the conclusions of Nelson et al. [J. Chem. Phys. 87, 6365 (1987)]. [ABSTRACT FROM AUTHOR]

The electric dipole moment of the linear molecular ion HN+2 was determined by measuring the isotope shifts of the rotational g factors of different isotopic species. We studied the Zeeman effect of the R(6) rotational transition at 650 GHz. In a magnetic field of 5.4 T the rotational transition split into two components, separated by 2.2–2.5 MHz. The gR factors were determined for H14N14N+, H14N15N+, and H15N14N+. The dipole moment for H14N14N+ was determined as (3.4±0.2) D, which is in excellent agreement with the theoretical value. The transitions were observed by direct absorption spectroscopy with a tunable FIR sideband spectrometer. The ions were generated in a modified anomalous discharge placed in a superconducting magnet. [ABSTRACT FROM AUTHOR]

Expressions are derived for the nuclear quadrupole splittings in the E3 and E4 (para–para) states of (NH3)2 and it is shown that these can be matched with the standard expressions for rigid rotors with two identical quadrupolar nuclei. The matching is exact only when the off-diagonal Coriolis coupling is neglected. However, the selection rules for rotational transitions are just opposite to those for the rigid rotor. Hyperfine splittings are measured for the J=2←1 transitions in the E3 and E4 states with |K|=1; the quadrupole coupling constants χaa=0.1509(83) MHz and χbb-χcc=2.8365(83) MHz are extracted from these measurements by the use of the above mentioned correspondence with the rigid rotor expressions. The corresponding results are also calculated, with and without the Coriolis coupling, from the six-dimensional vibration–rotation–tunneling (VRT) wave functions of (NH3)2, which were previously obtained by Olthof et al. [E.H.T. Olthof, A. van der Avoird, and P.E.S. Wormer, J. Chem. Phys. 101, 8430 (1994)]. From the comparison of χaa with the measured value it follows that the semiempirical potential and the resulting VRT states of Olthof et al. are very accurate along the interchange ([variant_greek_theta]A,[variant_greek_theta]B) coordinate. From χbb-χcc it follows that this potential is probably too soft in the dihedral angle γ=γA-γB, which causes the torsional amplitude to be larger than derived from the experiment. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c-type) at 21 cm-1, and to vb1 =1+ (b-type) at 25 cm-1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in the range 2.5–7 cm-1 and the lower tunneling component of the excited bending vibration lies between 39 and 43 cm-1 above the ground state of the complex. The experimentally determined center-of-mass separation (Rc.m. =3.75 Å) and harmonic stretching force constant (ks =0.0134 mdyn/Å) are compared to those of related first and second row hydrides. The large amplitude motions occurring within this complex make it difficult to establish its structure. [ABSTRACT FROM AUTHOR]