ALBERT

Notizen: 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 "strong collision" regime, i.e., one in which the branching ratios of the rotational relaxation do not depend on the initial rotational state. For small droplets we speculate that, because of finite size effects, the density of (surface) states may become discontinuous, producing deviations from the "thermal" behavior of the larger systems. © 2000 American Institute of Physics.

Notizen: Three-body interactions in a homonuclear van der Waals bound trimer (the 1 4A2′ state of Na3) are studied spectroscopically for the first time using laser induced emission spectroscopy on a liquid helium nanodroplet coupled with ab initio calculations. The van der Waals bound, spin polarized sodium trimers are prepared via pickup by, and selective survival in, a beam of helium clusters. Laser excitation from the 1 4A2′ to the 2 4E′ state, followed by dispersion of the fluorescence emission, allows for the resolution of the structure due to the vibrational levels of the lower state and for the gathering of precise information on the three-body interatomic potential. From previous experiments on Na2 we know that the presence of the liquid helium perturbs the spectra by a very small amount [see J. Higgins et al., J. Phys. Chem. 102, 4952 (1998)]. Ab initio potential energy calculations are carried out at 42 geometries of the lowest quartet state using the coupled cluster method at the single, double, and noniterative triple excitations level [CCSD(T)]. The full potential energy surface is obtained from the ab initio points using an interpolation procedure based on a Reproducing Kernel Hilbert Space (RKHS) methodology. This surface is compared to a second, constructed using an analytical model function for both the two-body interaction and the nonadditivity correction. The latter is calculated as the difference between the CCSD(T) points and the sum of the two-body interactions. The bound vibrational states are calculated using the two potential energy surfaces and are compared to the experimentally determined levels. The calculated bound levels are combined with an intensity calculation of the ν2″ mode of E′ symmetry derived from a Jahn–Teller analysis of the excited electronic state. The calculated frequencies of ν1″ and ν2″ are found to be 37.1 cm−1 and 44.7 cm−1, respectively, using the RKHS potential surface while values of 37.1 cm−1 and 40.8 cm−1 are obtained from the analytical potential. These values are found to be in good to fair agreement with those obtained from the emission spectrum and to be significantly different from any values calculated from additive potential energy surfaces. The 1 4A2′ Na3 potential energy surface is characterized by a D3h symmetry minimum of −850 cm−1 (relative to the three 3 2S Na atom dissociation limit) with a bond distance of 4.406 Å. This bond distance differs by about 0.8 Å from the value of 5.2 Å found for the sodium triplet dimer. This means that approximately 80% of the binding energy at the potential minimum is due to three-body effects. This strong nonadditivity is overwhelmingly due to the deformability of the valence electron density of the Na atoms which leads to a significant decrease of the exchange overlap energy in the trimer. © 2000 American Institute of Physics.

Notizen: The purely rotational microwave spectrum of HCN and DCN embedded in 4He nanodroplets has been measured. The J=0→1 transitions for both molecules have been recorded at 72.21 and 59.90 GHz, respectively. The increase in moment of inertia due to the presence of liquid helium, which the assumption of adiabatic following of the helium density predicts should be almost identical for both molecules, is found to be 9% smaller for the faster of the two rotors, HCN. This result is interpreted as a breakdown of the adiabatic following approximation, which is valid for the slower rotors. Power-saturation measurements have also been performed, and show that the rotational relaxation time for these molecules is on the order of 10−8 s. © 2000 American Institute of Physics.

Notizen: High-resolution helium nanodroplet isolation spectra of the first overtone (2ν1) of the acetylenic stretch of several substituted acetylenes (RC≡C–H) at T=0.38 K, have been observed for the first time. A tunable 1.5 μm laser is coupled, using a power buildup cavity, to a beam of He droplets seeded with the molecule to be studied. Absorption spectra are recorded by monitoring the beam depletion as a function of laser frequency with a thermal detector. The spectra of hydrogen cyanide (HCN), monodeuteroacetylene (DCCH), cyanoacetylene (NCCCH), propyne (CH3CCH), trifluoropropyne (CF3CCH), 3,3-dimethylbutyne ((CH3)3CCCH), and trimethylsilylacetylene ((CH3)3SiCCH) have been recorded. Due to the superfluid nature of the droplet, rotational resolution is achieved despite the presence of some solvent-induced broadening. The spectroscopic constants have been extracted by means of spectral simulations. The resulting rotational constants are smaller than for the bare molecule by a factor which depends on the molecule nonsphericity and its gas-phase moment of inertia. The linewidths are found to be at least twice as large as those of the corresponding fundamental (ν1) transitions observed in a helium droplet by Nauta et al. [Faraday Discuss. Chem. Soc. 113, 261 (1999) and references therein]. The helium-induced spectral shifts are found to be very small, but cannot be easily rationalized. © 2000 American Institute of Physics.

Notizen: We have used infrared–infrared double resonance spectroscopy to record a rovibrational eigenstate resolved spectrum of benzene in the region of the CH stretch first overtone. This experiment is the first of a series aimed at investigating intramolecular vibrational energy redistribution (IVR) in aromatic molecules. The experiment has been carried out in a supersonic molecular beam apparatus using bolometric detection. A tunable resonant cavity was used to enhance the on-beam intensity of the 1.5 μm color center laser used to pump the overtone, and a fixed frequency [R(30)] 13CO2 laser was used to saturate the coinciding ν18 rQ(2) transition of benzene. After assigning the measured lines of the highly IVR fractionated spectrum to their respective rotational quantum number J, analysis of the data reveals that the dynamics occurs on several distinct time scales and is dominated by anharmonic coupling with little contribution from Coriolis coupling. After the fast (∼100 fs) redistribution of the energy among the previously observed "early time resonances" [R. H. Page, Y. R. Shen, and Y. T. Lee, J. Chem. Phys. 88, 4621 (1988) and 88, 5362 (1988)], a slower redistribution (10–20 ps) takes place, which ultimately involves most of the symmetry allowed vibrational states in the energy shell. Level spacing statistics reveal that IVR produces a highly mixed, but nonstatistical, distribution of vibrational excitation, even at infinite time. We propose that this nonintuitive phenomenon may commonly occur in large molecules when the bright state energy is localized in a high-frequency mode. © 2000 American Institute of Physics.

Notizen: We have measured the laser-induced fluorescence excitation spectra of the 3 1P10←3 1S0 transition of Mg atoms solvated in helium nanodroplets. The observed blue shifts and line broadenings mirror the shifts and broadenings observed in studies of Mg atoms solvated in bulk liquid helium. This similarity allows us to conclude that Mg atoms reside in the interior of the helium droplet. The 3 1P10←3 1S0 transition shows a splitting which we attribute to a quadrupolelike deformation of the cavity which forms around the solute atom after excitation. Temporal evolution of the fluorescence from the solvated 3 1P10 Mg yields a longer lifetime (2.39±0.05 ns) than found in vacuum (1.99±0.08 ns). This difference can be accounted for quantitatively by evaluation of the anisotropic distribution of the helium density in the neighborhood of the excited Mg atom. The question of solvation vs surface location for the guest atoms is also discussed in light of the model of Ancilotto et al. [F. Ancilotto, P. B. Lerner, and M. W. Cole, J. Low Temp. Phys. 101, 1123 (1995)], of existing metal atom–helium potential energy functions, and of our own calculations for the MgHe and CaHe ground states. While the Ancilotto model successfully predicts solvation (or lack of it) if the solvation parameter of the guest atom is not too near the threshold of 1.9, the present knowledge of the interatomic potentials is not precise enough to test the model in the neighborhood of the critical value. © 2000 American Institute of Physics.

Notizen: The intramolecular vibrational relaxation (IVR) of an excited Si–H stretch (second overtone) and C–H stretch (first overtone) in methylsilane has been examined by eigenstate resolved infrared spectroscopy. The experiment probes a molecular beam produced in a supersonic expansion, excited by a laser in a power buildup cavity, and detected by a liquid helium cooled silicon bolometer. The Si–H stretch [local mode (3,0,0), both A and E combinations] is compared with the nearly isoenergetic C–H stretch [predominantly the 2ν70 band]. With the calculated density of states almost unchanged, the two modes exhibit very different IVR behavior, which is quantified in terms of the lifetime of the bright states and the coupling between the bright states and the dark states. © 1997 American Institute of Physics.

Notizen: A molecular beam spectrometer capable of achieving sub-Doppler resolution at 2 eV (∼18 000 cm−1) of vibrational excitation is described and its performance demonstrated using the CH stretch chromophore of HCN. Two high finesse resonant power-buildup cavities are used to excite the molecules using a sequential double resonance technique. A v=0→2 transition is first saturated using a 1.5 μm color center laser, whereupon a fraction of the molecules is further excited to the v=6 level using an amplitude modulated Ti:Al2O3 laser. The energy absorbed by the molecules is detected downstream of both excitation points by a cryogenically cooled bolometer using phase sensitive detection. A resolution of approximately 15 MHz (i.e., three parts in 108) is demonstrated by recording a rotational line in the v=6 manifold of HCN. Scan speeds of up to several cm−1/h were obtained, with signal-to-noise ratios in excess of 100. The high signal-to-noise ratio and a dynamic range of 6×104 means that future experiments to study statistical intramolecular vibrational energy redistribution in small molecules and unimolecular isomerizations can be attempted. We would also like to point out that, with improved metrology in laser wavelengths, this instrument can also be used to provide improved secondary frequency standards based upon the rovibrational spectra of molecules. © 2000 American Institute of Physics.

Notizen: Helium nanodroplet isolation is used to produce van der Waals-bound quartet state alkali trimers (Na3 and K3) selectively over the corresponding chemically bound doublet trimers. Frequency-resolved excitation and emission spectroscopy reveals the presence of nonadiabatic spin–flip processes in the electronically excited states. A total of four quartet to quartet electronic transitions are observed: the 2 4E′,1 4E←1 4A2′ transitions of Na3 and the 1 4A1″,2 4E′←1 4A2′ transitions of K3. Time-resolved spectroscopy reveals that intersystem crossing times in Na3 decrease from 1.4 ns after excitation of the 0–0 band to approximately 400 ps for the higher vibronic levels (3,5/2). Analysis of the resonant quartet fluorescence reveals that the excited electronic state cools vibrationally on a time scale that is comparable to, but slower than, the intersystem crossing time. © 2001 American Institute of Physics.