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Kevin K Lehmann

Publications and source records attributed to Kevin K Lehmann.

8 recordsLinked to original sources
PubMed2004

UV spectra of benzene isotopomers and dimers in helium nanodroplets.

We report spectra of various benzene isotopomers and their dimers in helium nanodroplets in the region of the first Herzberg-Teller allowed vibronic transition 6(0)(1) (1)B(2u)<--(1)A(1g) (the A(0) (0) transition) at approximately 260 nm. Excitation spectra have been recorded using both beam depletion detection and laser-induced fluorescence. Unlike for many larger aromatic molecules, the monomer spectra consist of a single "zero-phonon" line, blueshifted by approximately 30 cm(-1) from the gas phase position. Rotational band simulations show that the moments of inertia of C(6)H(6) in the nanodroplets are at least six-times larger than in the gas phase. The dimer spectra present the same vibronic fine structure (though modestly compressed) as previously observed in the gas phase. The fluorescence lifetime and quantum yield of the dimer are found to be equal to those of the monomer, implying substantial inhibition of excimer formation in the dimer in helium.

Journal Article↗
PubMed2004

Cavity ringdown strain gauge.

Biconical tapered single-mode fiber, which is common in many telecommunications components, offers an alternative sensor to typical optical fiber strain gauges that are susceptible to temperature and pressure effects and require expensive and sophisticated signal acquisition systems. Cavity ringdown spectroscopy, a technique commonly applied to high-sensitivity chemical analysis, offers detection sensitivity advantages that can be used to improve strain measurement with biconical tapers. Combining these two technologies in a spatially extended resonator, we demonstrate a minimum detectable change in ringdown time of 0.08%, corresponding to a minimum detectable displacement of 4.8 nm, and a sensitivity to strain as small as 79 n epsilon/square root(Hz) over a 5-mm taper length.

Journal Article↗
PubMed2004

Evaporative cooling of helium nanodroplets with angular momentum conservation.

Evaporative cooling of helium nanodroplets is studied with a statistical rate model that includes, for the first time, angular momentum conservation. While the final temperature of the droplets is almost identical to that previously predicted and later observed, the distribution of total droplet energy and angular momentum states is vastly more excited than a canonical distribution at the same temperature. It is found that the final angular momentum of the droplets is highly correlated with the initial direction, and that a significant fraction of the alignment of the total angular momentum should be transferred to the rotational angular momentum of an embedded molecule.

Journal Article↗
PubMed2004

Spectroscopy of highly excited vibrational states of HCN in its ground electronic state.

An experimental technique based on a scheme of vibrationally mediated photodissociation has been developed and applied to the spectroscopic study of highly excited vibrational states in HCN, with energies between 29,000 and 30,000 cm(-1). The technique consists of four sequential steps: in the first one, a high power laser is used to vibrationally excite the sample to an intermediate state, typically (0,0,4), the nu3 mode being approximately equivalent to the C-H stretching vibration. Then a second laser is used to search for transitions between this intermediate state and highly vibrationally excited states. When one of these transitions is found, HCN molecules are transferred to a highly excited vibrational state. Third, a ultraviolet laser photodissociates the highly excited molecules to produce H and CN radicals in its A 2Pi electronic state. Finally, a fourth laser (probe) detects the presence of the CN(A) photofragments by means of an A-->B-->X laser induced fluorescence scheme. The spectra obtained with this technique, consisting of several rotationally resolved vibrational bands, have been analyzed. The positions and rotational parameters of the states observed are presented and compared with the results of a state-of-the-art variational calculation.

Computer Simulation↗
PubMed2004

Bias in the temperature of helium nanodroplets measured by an embedded rotor.

The rovibrational spectra of molecules dissolved in liquid 4He nanodroplets display rotational structure. Where resolved, this structure has been used to determine a temperature that has been assumed to equal that of the intrinsic excitations of the helium droplets containing the molecules. Consideration of the density of states as a function of energy and total angular momentum demonstrates that there is a small but significant bias of the rotor populations that make the temperature extracted from a fit to its rotational level populations slightly higher than the temperature of the ripplons of the droplet. This bias grows with both the total angular momentum of the droplet and with the moment of inertia of the solute molecule.

Journal Article↗
PubMed2003

Trace moisture detection using continuous-wave cavity ring-down spectroscopy.

We have developed an instrument to measure trace concentrations of small hydride species in gases using continuous-wave cavity ring-down spectroscopy with near-infrared diode laser excitation. An rms baseline equivalent absorbance of 9.2 x 10(-11) cm(-1)/square root(n) is found, where n is the number of ring-down transients. When the 1396.376-nm absorption line of water is used, this corresponds to a noise equivalent moisture concentration in nitrogen gas of 68 pptv/square root(n). Water vapor concentration is detected over a range extending from 3 to 1000 ppbv and found to depend linearly on the concentration as determined by a calibrated commercial moisture sensor.

Journal Article↗
PubMed2002

Calculation of hydrodynamic mass for atomic impurities in helium.

We present a simple numerical procedure for calculating the irrotational hydrodynamic flow around a spherical solute in superfluid helium. The results for alkali cations are compared to recent many-body variational Monte Carlo (VMC) calculations. The helium contribution to the effective masses calculated by the VMC and hydrodynamic methods are 12.9(4.6) versus 9.4 u for Li+, 48.2(5.6) versus 52.1 u for Na+, 69.6(4.8) versus 70.1 u for K+, and 6.4(8.8) versus 6.8 for Cs+. In all cases, the agreement of the two results are in within the error estimate of the VMC calculation, demonstrating the accuracy of hydrodynamic treatment of helium motion even on the atomic length scale.

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