Infrared Spectroscopy of Size-Selected Water and Methanol Clusters

Infrared molecular beam depletion and fragment spectroscopy has been employed to study the absorption behavior of small water clusters [(H2O)n, n=2,3,4,5]. The spectral region between 3300 and 3800 cm−1 was covered with an injection‐seeded optical parametric oscillator. Size‐specific information has been obtained by dispersing the cluster beam with a secondary helium beam and measuring the depletion as a function of the scattering angle. Three absorption bands could be assigned to the water dimer (H2O)2, with the bonded OH stretch being localized at 3601 cm−1. For each of the larger water clusters (n=3,4,5), which have cyclic structures, two absorption bands could be identified, one belonging to the free OH stretch and the other being due to the excitation of the OH ring vibration. The measurements on free water clusters were complemented by studies on small water complexes formed on large argon clusters. The positions of the absorption bands observed in these spectra are close to those found for (H2O)n i...

Infrared spectroscopy of small size‐selected water clusters

Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters upon excitation of the O–H stretch vibration at 2.7 μ. The tunable infrared radiation has been obtained from a Nd:YAG laser pumped optical parametric oscillator. Pure methanol dimer spectra, without contamination from larger clusters, have been measured using the scattering selection technique or working at reduced temperatures. The dimer spectrum features two absorption peaks at 3574.4 and 3684.1 cm−1. The lower frequency peak is redshifted by 106.6 cm−1 from the monomer value and is assigned to the excitation of the proton donor in the hydrogen bonded complex. The proton acceptor peak is blueshifted by only 3.1 cm−1. Methanol trimer dissociation is observed at 3462 cm−1, whereas larger clusters absorb at still lower frequencies. Absolute photodissociation cross sections were measured for both methanol dimer bands. The integrated dissociation cross sections were 5.59(25)×10−21 cm2 /molecule and 7.1(2.2)×10−22 cm2 /molecule for the proton donor and acceptor bands, respectively. The integrated dissociation cross sections are directly related to the O–H line strengths in the dimer. The increased line strength, for the proton donor band relative to the acceptor band by a factor of ∼8, is in accord with previous observations associated with hydrogen bond formation.

Dissociation of small methanol clusters after excitation of the O–H stretch vibration at 2.7 μ

Potential energy, electric dipole moment, and electronic transition moment surfaces have been calculated for the A and X states of NH3 from CASSCF and CEPA electronic wave functions. Anharmonic vibrational term values, Franck–Condon factors, and A–X radiative transition probabilities for the symmetric stretching and bending modes of NH3 and ND3 have been evaluated. The theoretical absorption spectra at room and low temperatures agree well with experimental data. The symmetric stretching mode in the A state has only small intensities in the A–X absorption spectrum. Emission rates from various initial vibronic levels of the A state are given. The ab initio electric dipole moment surfaces for the ground state of NH3 have been used to compute transition moments, which are in good agreement with experimental data.

Theoretical A 1A‘2–X 1A1 absorption and emission spectrum of ammonia

We have measured and assigned more than 800 new far-infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far-infrared and microwave spectra for this cluster. The vibration–rotation–tunneling (VRT) states of the ammonia dimer connected by electric-dipole-allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A+A states (states formed from the combination of two ammonia monomers in A states), A+E states, and E+E states. We present complete experimentally determined energy-level diagrams for the Ka=0 and Ka=1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G144. This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the "donor" and "acceptor" roles of the monomers, internal rotation of the monomers about their C3 symmetry axes, and quite unexpectedly, "umbrella" inversion tunneling.In the Ka=0 A+E and E+E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In Ka=1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm^–1, respectively. In the A+E states, the measured 20.5 cm^–1 splitting can result from a difference in "donor" and "acceptor" internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far-infrared subbands is the very-low-frequency out-of-plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990); M. J. Frisch, J. E. Del Bene, J. S. Binkley, and H. F. Schaefer III, ibid. 84, 2279 (1986)]. In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that "donor–acceptor" interchange tunneling was nearly quenched in the ammonia dimer [D. D. Nelson, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 83, 6201 (1985); D. D. Nelson, W. Klemperer, G. T. Fraser, F. J. Lovas, and R. D. Suenram, ibid. 87, 6364 (1987)]. Our experimental results, considered together with the results of six-dimensional calculations of the VRT dynamics presented by van Bladel et al. in the accompanying paper [J. Chem. Phys. 97, 4750 (1992)], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.

/pdf/multidimensional-hydrogen-tunneling-dynamics-in-the-ground-2ar8osh8os.pdf

Multidimensional hydrogen tunneling dynamics in the ground vibrational state of the ammonia dimer

Infrared photodissociation of size-selected small ammonia clusters

A steady supersonic molecular beam of internally cold (C2H4)n clusters is crossed by the IR radiation of a pulsed CO2 laser. Subsequent scattering by a secondary He beam disperses the cluster beam and allows unequivocal detection of selected cluster species which is not disturbed by fragmentation processes in an ionizer. The dependence of IR photon absorption on cluster size is investigated in the region of the ν7 vibration of C2H4; predissociation spectra are recorded for (C2H4)n complexes with n=2, 3, 4, and 5. The absorption profiles are structureless and all have, regardless of cluster size, the same width (FWHM=12 cm−1) and peak at the same position (ν0=953 cm−1). From the dependence of the dissociation yield on the laser fluence, transition probabilities are determined for n=2, 3, and 4.

Infrared photodissociation and cluster‐specific detection of internally cold (C2H4)n van der Waals complexes

Coherent anti-stokes Raman spectroscopy of ammonia in supersonic molecular beams

Small ethylene clusters (C2H4)n are dissociated upon absorption of a C02-laser photon by exciting the υ7-mode of the monomer. The clusters are selected according to their size by scattering them from a helium beam. Since an appreciable amount of energy is transferred to the cluster during the collision, the laser photons interact with internally cold or hot clusters depending on whether the interaction is before or after the collision center. Measurements of the frequency and fluence dependences of the photodissociation cross section are carried out for cold and hot clusters from n=2 to n=6. The spectra of the cold clusters show the same shape and line- width for all cluster sizes investigated, while those of the hot clusters are characterized by large variations in shape and line- width. The very structured dimer spectrum is explained by the excitation of hot bands.

Infrared photodissociation of van der Waals complexes selectively prepared by molecular beam scattering

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Infrared photodissociation of size-selected small ammonia clusters

Infrared photodissociation and cluster‐specific detection of internally cold (C2H4)n van der Waals complexes

Coherent anti-stokes Raman spectroscopy of ammonia in supersonic molecular beams

Infrared photodissociation of van der Waals complexes selectively prepared by molecular beam scattering