K. Nauta

Solvent mediated vibrational relaxation: Superfluid helium droplet spectroscopy of HCN dimer

Rotationally resolved infrared spectra are reported for the HCN dimer, grown and solvated in liquid helium droplets. This is the first study for which two different vibrational modes within the same liquid helium solvated molecule have been observed, namely those associated with the “free” and the “hydrogen-bonded” C–H stretching vibrations. Comparing the line broadening in these two bands, we conclude that the helium solvent plays an important role in the vibrational relaxation dynamics of the dimer. The rotational constants obtained from these spectra indicate that the dimer rotates more slowly in the liquid than in the gas phase.

Rotational and vibrational dynamics of CO2 and N2O in helium nanodroplets

Infrared spectra are reported for carbon dioxide and nitrous oxide solvated in superfluid helium droplets, corresponding to the vibrational excitation of the (0201)/(1001) Fermi diad. Although the rotational constants of these two molecules are similar in the gas phase, they are observed to be quite different in liquid helium, namely, 0.154 cm−1 for CO2 and 0.0717 cm−1 for N2O. In addition, solvation in helium results in shifts in the vibrational origin that are in the opposite directions, −0.42 cm−1, for CO2 and +1.2 cm−1 for N2O. The spectra also show strong droplet size dependence, indicative of the interactions between the molecule and the liquid.

Metastable vibrationally excited HF (v=1) in helium nanodroplets

High-resolution infrared laser spectroscopy is used to study hydrogen fluoride solvated in helium nanodroplets. The results clearly show that the vibrationally excited HF (v=1) does not relax on the time scale of the experiments (0.5 ms) and that the large linewidth of the R(0) transition (0.43 cm−1) results from rotational relaxation. A large dc electric field is applied to induce a Q(0) transition, providing an accurate value for the rotational constant of HF in solution (19.47 cm−1), only 2% smaller than in the gas phase.

Finite size effects and rotational relaxation in superfluid helium nanodroplets: Microwave-infrared double-resonance spectroscopy of cyanoacetylene

Microwave-infrared double-resonance spectroscopy has been used to probe the solvation environment and its influence on the rotational relaxation of a cyanoacetylene molecule embedded in a superfluid 4He nanodroplet. The results support a model in which (within any given rotational state) the guest molecules are distributed over a set of spectroscopically inequivalent states which are most likely “particle-in-a-box” states originating from the confinement of the guest molecule within the droplet. Revisitation of previously collected microwave–microwave double-resonance data suggests that transitions between these states occur at a rate which is comparable to the rotational relaxation rate, but not fast enough as to produce motionally narrowed, homogeneous absorption lines. The relative intensities of the rotational lines in the microwave-infrared double-resonance spectra are observed to depend strongly on the average droplet size. In the large droplet limit we can explain the observed pattern by invoking a...

The hydrogen fluoride dimer in liquid helium: A prototype system for studying solvent effects on hydrogen bonding

High-resolution infrared spectra are reported for the “free” and “hydrogen bonded” H–F stretches of the hydrogen fluoride dimer solvated in helium nanodroplets. These rotationally resolved spectra provided detailed information concerning the effect of the helium solvent on the vibrational frequencies, rotational constants and tunneling dynamics of the dimer. The end-over-end rotation of the dimer is slowed by a factor of 2.2 by the helium, while the faster rotation about the a axis remains essentially unaffected. The interchange tunneling is reduced significantly (∼40%) in both the ground and vibrationally excited states. The effective tunneling barrier is higher than in the gas phase, making it easier to quench the tunneling motion with a large dc (direct current) electric field. Rapid rotational relaxation is observed from the Ka=1 state, resulting in significant broadening of the corresponding subband. Combination bands are observed for the intermolecular F–F stretch and trans-bend vibrations, providin...

The vibrational and rotational dynamics of acetylene solvated in superfluid helium nanodroplets

Infrared spectra are reported for 12C2H2, 13C2H2, 12C13CH2, and 12C2HD solvated in superfluid helium nanodroplets, corresponding to excitation of the C–H stretches. For the lowest rotational states (J=0 and J=1), molecular rotation is only weakly hindered by the liquid and the associated transitions are sharp, having small frequency shifts relative to the gas phase. In contrast, the R(1) transitions of 12C2H2 and 13C2H2, which access the corresponding J=2 rotational states, are much broader and more strongly shifted from the gas phase. The linewidths of the R(0) transitions show a strong isotopic dependence, which we take as evidence that vibrational resonances present in some of the isotopomers enhance the vibrational relaxation rate.

Infrared spectroscopy and structures of Arn–HF in liquid helium nanodroplets

Infrared spectra are reported for Arn–HF complexes up to the closing of the first solvent shell. These complexes were grown in superfluid helium droplets, leading to the formation of multiple isomers for n>3. The vibrational frequency shifts associated with the HF stretch are in excellent agreement with previous theoretical calculations. The relative abundances of the various isomers is found to be dependent upon the order of pick-up of the HF and argon. When HF is added to the droplets first the isomers corresponding to the solvated HF are produced in high abundance. Growth of the argon cluster prior to addition of the HF leads to the latter being on the “surface” of the argon cluster.

Rotationally resolved infrared spectroscopy of h2- and d1-formic acid monomer in liquid He droplets

Rotationally resolved spectra of h2- and d1-formic acid embedded in liquid helium droplets have been recorded in the spectral region of the O–H (ν1) and C–H (ν2) stretch vibrations. Accidental resonant mixing between the ν1-band of h2-formic acid and the combination bands (ν2+ν7) and (ν2+ν9) has been observed. The fundamental ν1 band can interact via Fermi resonance with the (ν2+ν7), and the (ν2+ν7) via Coriolis coupling with the (ν2+ν9) band. Examination of the resonance induced line-broadening effects suggests that the helium environment modifies vibrational relaxation dynamics. The 2ν3 C=O stretch overtone is also observed and was assigned by a density functional theory (DFT) ab initio calculation. The spectroscopic constants are determined by fitting all spectra with a standard gas phase Hamiltonian.

The rotational dynamics of N2–HF and OC–HF in helium nanodroplets

Infrared spectra are reported for the N2–HF and OC–HF complexes formed in helium nanodroplets. The spectra are rotationally resolved and show that the structures of the complexes are essentially the same (linear) as determined previously from gas phase studies. The rotational constants of the helium solvated complexes are smaller than the gas phase values, in agreement with what has been previously observed for several other systems. N2–HF is particularly interesting when studied in the presence of a large electric field, given that the change in the dipole moment upon vibrational excitation is large, so that the individual transitions in the pendular spectrum can be resolved. This enables us to monitor the change in the helium environment around the molecule as it goes from rotational to pendular motion. The results also reveal that the relaxation lifetime of these pendular states in helium is comparable to the flight time of the droplets through the electric field and depends upon the droplet size.

Vibrational relaxation of Ne, Ar, Kr–HF (v=1) binary complexes in helium nanodroplets

We report high-resolution infrared laser spectra of the Ne, Ar, Kr–HF binary complexes solvated in liquid helium droplets. The present experiments show that the vibrational relaxation rates become progressively faster in the series Ne, Ar, and Kr–HF. Ar–HF is an intermediate case, with vibrational relaxation occurring on the time scale of the flight time of the droplets through the apparatus (0.5 ms). In this case, the data show that the relaxation rate is also dependent upon the size of the helium droplet in which the complex is solvated.