L. L. Connell

Rotational coherence spectroscopy and structure of phenol dimer

Rotational coherence spectroscopy has been used to measure the rotational constants of four isotopomers of phenol dimer and a single isotopomer of p‐cresol dimer. From the results of these measurements, together with spectroscopic results reported by others, a geometry for phenol dimer is deduced. The species is found to be bound by an O–H⋅⋅⋅O hydrogen bond. The orientation of the phenyl moieties is such that they make maximal contact consistent with the constraints imposed by the hydrogen bond and by the van der Waals radii of the atoms. This geometric feature is cited as evidence for the significance of aromatic–aromatic attraction in the intermolecular interaction between the phenols.

Conformational analysis of jet-cooled tryptophan analogs by rotational coherence spectroscopy

Abstract Excited state rotational constants obtained by rotational coherence spectroscopy are reported for different conformers of jet-cooled tryptamine, 3-indole acetic acid, and 3-indole propionic acid. From the measured constants and information from other sources, conclusions about the structures of the conformers are made.

Theory of rotational coherence spectroscopy as implemented by picosecond fluorescence depletion schemes

We present a perturbation theory analysis of four time‐resolved fluorescence depletion schemes that are useful, or potentially useful, in rotational coherence spectroscopy. The analysis shows that ground‐state rotational constants determine the rotational coherence effects in fully resonant, time‐resolved stimulated Raman‐induced fluorescence depletion (TRSRFD), excited‐state rotational constants determine such effects in time‐resolved stimulated emission spectroscopy (TRSES), and both ground‐ and excited‐state constants do so in time‐resolved fluorescence depletion (TRFD). An analysis of a variant of the TRSRFD scheme in which the stimulated Raman process is not resonance‐enhanced shows that this method gives rise to qualitatively different rotational coherence effects than fully resonant TRSRFD. It is argued that the scheme may, nevertheless, be a viable means of ground‐state rotational coherence spectroscopy. Expressions for the calculation of rotational coherence effects in TRFD, TRSRFD, and TRSES tra...

Ground-state rotational coherence spectroscopy of jet-cooled molecules by stimulated Raman-induced fluorescence depletion

Abstract We report the demonstration of a new technique of rotational coherence spectroscopy that applies to the time-domain measurement of ground-state rotational constants. The technique, stimulated Raman-induced fluorescence depletion, is a picosecond pump-probe scheme in which ground-state rotational coherences prepared via stimulated Raman scattering by the pump pulse are probed via fluorescence induced by the vibronically resonant, variably delayed probe pulse. The scheme is demonstrated with experiments on jet-cooled t -stilbene. The potential of the method and its relation to other techniques are discussed.

Structural characterization of clusters of perylene with argon and neon by rotational coherence spectroscopy

Abstract Rotational coherence spectroscopy has been used to measure rotational constants of the perprotonated and perdeuterated isotopomers of perylene—Ar, two isomers of perylene—(Ar)2, perylene—Ne, and perylene—(Ne)2. A structural analysis of the various species has been performed based on these results. A comparison of the geometries so obtained with those proposed by others based on potential energy calculations and vibronic frequency shifts confirms the validity of the latter approach for perylene—(rare gas)n species.

The geometry of carbazole-(Ar)2 from rotational coherence spectroscopy

Abstract Rotational coherence spectroscopy has been used to investigate the geometry of the carbazole—(Ar) 2 cluster. The data strongly indicate that the structure has one argon atom above and the other below the central ring of the carbazole moiety. This result is contrary to the minimum-energy structure predicted by calculations based on a widely used intermolecular potential energy function. Possible reasons for this discrepancy are discussed.

Rotational coherence spectroscopy and structure of the perylene-benzene van der Waals complex

Abstract Rotational coherence spectroscopy has been used to measure rotational constants for four isotopomers of the aromatic-aromatic dimer perylene-benzene. Possibilities for the vibrationally averaged dimer geometry have been deduced from the measured values. In the geometries the benzene moiety is close to centrally bound to the perylene and such that its ring plane is parallel or nearly parallel to the plane of the perylene. The geometries are discussed in terms of both the intermolecular forces between the species and the structural results for other aromatic dimers.

Fourier transform stimulated emission pumping spectroscopy

Abstract Theoretical and experimental results that demonstrate a new technique of non-linear interferometry based on stimulated emission pumping spectroscopy (SEPS) are presented. It is shown that splittings between the initial and final states in SEP processes can be measured by the method. Advantages and disadvantages of the technique relative to spectral domain SEPS are discussed.

Structural study of the hydrogen-bonded 1-naphthol⋅(NH3)2 cluster

The structure of the 1-naphthol⋅(NH3)2 cluster was investigated by rotational coherence spectroscopy (RCS), mass selective one- and two-color resonant two-photon ionization (R2PI) experiments and ab initio calculations. RCS measurements yielded rotational constants of 1-naphthol⋅(NH3)2 as A=1197, B=500, and C=413 MHz, as well as those for several isotopomers. The counterpoise-corrected second-order Moller–Plesset perturbation theory (MP2) method predicts two isomers A and B. Both structures have hydrogen bonded naphthol–OH⋯NH3⋯NH3 chains, with the second NH3 bent above the proximal aromatic ring and pointing towards the π-electron system and have nearly the same binding energy. The experimental rotational constants agree better with those calculated for structure B. The B3LYP and PW91 density functional methods also predict two isomers A, B with the rotational constants of B in acceptable agreement with experiment. Based on two-color R2PI experiments using low ionization frequency to suppress cluster frag...

High resolution Fourier transform stimulated emission and molecular beam hole‐burning spectroscopy with picosecond excitation sources: Theoretical and experimental results

Theoretical and experimental results relating to the picosecond laser implementation of Fourier transform stimulated emission spectroscopy (FT‐SES) and Fourier transform hole‐burning spectroscopy (FT‐HBS) in molecular beams are presented. It is shown that the resolution in the schemes is only limited by the length of the interferogram taken, and not by factors such as the excitation pulsewidths, excitation bandwidths, or the delay between the pump and probe laser pulses. In addition, the factors which determine the homogeneous and Doppler broadening in FT‐SES and FT‐HBS spectra are examined. It is found that ground‐state damping factors determine the homogeneous broadening in FT‐SES and excited‐state damping factors determine such broadening in FT‐HBS. Doppler broadening in both of the methods is shown to be that associated with two‐photon resonant vibrational transition frequencies rather than one‐photon resonant vibronic transition frequencies. The characteristics of the Fourier transform methods are co...