Kevin K. Lehmann

Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results

In this article, recent developments in helium nanodroplet isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. We discuss how molecular beam spectroscopy and matrix isolation spectroscopy can be usefully combined into a method that provides a unique tool to tackle physical and chemical problems which had been outside our experimental possibilities. Next, in reviewing the experimental methodology, we present design criteria for droplet beam formation and its seeding with the chromophore(s) of interest, followed by a discussion of the merits and shortcomings of radiation sources currently used in this type of spectroscopy. In a second, more conceptual part of the review, we discuss several HENDI issues which are understood by the community to a varied level of depth and precision. In this context, we show first how a superfluid helium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its ani...

Ring-down cavity absorption spectroscopy of the very weak HCN overtone bands with six, seven, and eight stretching quanta

A nonstandard, high sensitivity, absorption detection technique has been applied to the investigation of the very weak fifth, sixth, and seventh overtones of HCN at 100 Torr and 296 K. The frequency range covered is from 17 500 to 23 000 cm−1. We report high resolution, absolute absorption spectra with a noise equivalent sensitivity as low as ∼2×10−9/cm (recently improved to 7×10−10/cm). Band origins, rotational constants, and band intensities are reported and compared with calculated values. The HCN overtone spectra in the present study are not affected by any kind of perturbation, despite the high excitation energy involved.

Spectroscopy and dynamics in helium nanodroplets

This article provides a review of recent work in the field of helium nanodroplet spectroscopy with an emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.

On the relation of Child and Lawton’s harmonically coupled anharmonic–oscillator model and Darling–Dennison couplinga)

Child and Lawton’s1 model for two identifical coupled anharmonic oscillators is further discussed. It is shown that the effective Hamiltonian of that model can be transformed into a special case of the familiar Darling‐Dennison resonance Hamiltonian. (AIP)

The high‐resolution visible overtone spectrum of acetylenea)

Direct, gas‐phase overtone spectra of states corresponding to five and six quanta of C–H stretch have been obtained for four isotopically substituted acetylenes in the region 14 900–18 500 cm−1. Peak positions of individual rotation–vibration lines were determined to a precision of about 0.003 cm−1. A total of 39 bands are analyzed: 16 for 12C2H2; 14 for 13C2H2; five for 12C13CH2; and four for 12C2HD. The rotational structure of the bands is fit to a semirigid rotor Hamiltonian. However, the bands suffer from severe rotational perturbations, with typical matrix elements being on the order of 0.3 cm−1 or less. The error in the determined band origins varies between 0.001 and 0.1 cm−1, depending on the degree of perturbation. Precision rotational constants are also determined for the observed bands. The vibrational term values cannot be adequately explained in terms of the usual slightly anharmonic normal mode expansion. Even though acetylene represents an intermediate case between the local mode and normal...

The superposition principle and cavity ring‐down spectroscopy

Cavity ring‐down is becoming a widely used technique in gas phase spectroscopy. It holds promise for further important extensions, which will lead to even more frequent use. However, we have found widespread confusion in the literature about the nature of coherence effects, especially when the optical cavity constituting the ring‐down cell is excited with a short coherence length laser source. In this paper we use the superposition principle of optics to present a general and natural framework for describing the excitation of a ring‐down cavity regardless of the relative values of the cavity ring‐down time, the input pulse coherence time, or the dephasing time of absorption species inside the cavity. This analysis demonstrates that even in the impulsive limit the radiation inside a high finesse cavity can have frequency components only at the natural resonance frequencies of the cavity modes. As an immediate consequence, a sample absorption line can be detected only if it overlaps at least one of the cavi...

Fourier transform spectra of overtone bands of HCN from 5400 to 15100 cm−1☆

Abstract The absolute intensities and vibration-rotation constants of 26 overtone and combination bands of HCN are reported. The dynamic range for the absolute intensity measurements is nearly one million to one. Absorption spectra of HCN from 5400 to 15100 cm −1 were obtained using the Fourier transform spectrometer at the Kitt Peak National Solar Observatory with optical path lengths up to 432 m. The frequencies of 1346 assigned HCN lines were used to derive vibration-rotation constants for 23 bands (14 Σ-Σ, 4 Π-Π, 4 Π-Σ, and 1 Σ-Π) of H 12 C 14 N, 3 bands (Σ-Σ) of H 13 C 14 N, and 2 bands (Σ-Σ) of H 12 C 15 N. These new band origin and rotational constant data have been combined with existing data given in the literature to derive an improved set of vibrational (ω, x , and y ) constants and rovibrational (α and γ) constants for HCN. The vibrational dependence of the centrifugal distortion constants has also been examined. One thousand thirtysix derived line areas were used to determine absolute intensities for all 28 bands. Four weak stretch-only bands were observed for the first time: the (300)-(000), the (201)-(000), the (301)-(000) and the (202)-(000). Such data should be an important aid in accurately determining the CN contribution to the potential and dipole moment functions. Finally, we present a comparison of 13 of the measured absolute overtone intensities (stretching states only) with recent ab initio results.

Photoinduced Chemical Dynamics of High-Spin Alkali Trimers

Nanometer-sized helium droplets, each containing about 104 helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.

Classical chaos and quantum simplicity: Highly excited vibrational states of HCN

Direct overtone spectra of H 12C 14N, H 13C 14N, and H 12C 15N have been measured between 15 000 and 18 500 cm−1 with a precision of 0.001 cm−1. These were obtained using intracavity photoacoustic spectroscopy, with a fully automated laser system. The spectra are unperturbed. The transition energies and rotational constants are in good agreement with predictions of first order anharmonic constants. Classical trajectories for HCN have been computed on the best experimentally parameterized potential, and found to be stochastic 12 990 cm−1 above the ground state. Quantal density of states were computed for HCN and show that if extensive vibrational coupling occurs, the observed states would be highly perturbed. The simplicity of the observed states is shown to be expected given a Franck–Condon type limitation on significantly perturbing states. The results show the inapplicability of classical dynamics for predicting the dynamics of molecular vibrations.

Potential of a neutral impurity in a large 4 He cluster

This paper presents an analysis of the motion of an neutral impurity species in a nanometer scale 4 He cluster, extending a previous study of the dynamics of an ionic impurity. It is shown that for realistic neutral impurity–He potentials, such as those of SF6 and OCS, the impurity is kept well away of the the surface of the cluster by long range induction and dispersion interactions with He, but that a large number of ‘particle in a box’ center of mass states are thermally populated. It is explicitly demonstrated how to calculate the spectrum that arises from the coupling of the impurity rotation and the center of mass motion, and it is found that this is a potentially significant source of inhomogeneous broadening in vibration–rotation spectra of anisotropic impurities. Another source of inhomogeneous broadening is the hydrodynamic coupling of the rotation of the impurity with the center of mass velocity. A quantum hamiltonian to describe this effect is derived from the classical hydrodynamic kinetic energy of an ellipsoid. Simple analytic expressions are derived for the resulting spectral line shape for an impurity in bulk He, and the relevant matrix elements derived to allow fully quantum calculations of the coupling of the center of mass motion and rotation for an impurity confined in a spherical He cluster. Lastly, the hydrodynamic contribution to the impurity effective moment of inertia is evaluated and found to produce only a minor fractional increase.