ALBERT

Notes: We have measured the fundamental acetylenic C–H stretch near 3.0 μm of (CH3)3C–C≡CH and (CH3)3Si–C≡CH using an optothermal, molecular beam spectrometer. We find that the individual R(J) lines of the hydrocarbon are Lorentzian with a FWHM of 800 MHz indicating statistical intramolecular vibrational relaxation (IVR) with a 400 ps lifetime. The R(J) lines of the silicon compound are clearly asymmetric and, in addition, show a FWHM of about 150 MHz indicating a much longer (〉2 ns) lifetime. The increase in IVR lifetime in the larger density of states molecule may be due to reduced kinetic coupling resulting from the heavier Si atom.

Notes: 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.

Notes: We recorded rovibrational spectra of the 006+ level of 12C2H2 and the 2131 11−1 level of 13C2H2 in the ground electronic state using a two-photon sequential double resonance technique with a resolution of 15 MHz. Owing to the g/u symmetry of acetylene, the levels that we observe are inaccessible from the ground state by single photon techniques, and observation of these levels is reported here for the first time. Upper state rotational constants were derived from whole band fits of the observed lines, and compare favorably with expected values. Both spectra exhibit signs of local perturbations, and a density of states analysis leads us to believe that we are observing couplings to the full density of vibrational states one would expect from acetylene in this energy region. Despite the high resolution of our spectrometer, and the high excitation energy, no evidence for acetylene hydrogen permutation exchange isomerization (which is predicted to proceed through the vinylidene minimum on the potential) has been observed, implying that the rate of exchange isomerization is more than four orders-of-magnitude below the rate predicted by RRKM (Rice, Ramsperger, Kassel, and Marcus) theory. © 2000 American Institute of Physics.

Notes: 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.

Notes: 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.

Notes: Dispersed emission spectra collected upon the 4 2P3/2,1/2←4 2S1/2 optical excitation of K atoms attached to helium nanodroplets include broad, structured, red-shifted features which are shown to be due to K*He exciplex formation, paralleling our former observation of Na*He [J. Reho, C. Callegari, J. Higgins, W. E. Ernst, K. K. Lehmann, and G. Scoles, Discuss. Faraday Soc. 108, 161 (1997)]. The exciplex formation is demonstrated by the agreement obtained in comparing the K*He A(1) 2Π→X(1) 2Σ emission spectra with the predictions derived from available ab initio potential energy surfaces. Recent analysis of both exciplex emissions also points to the possibility of triatomic (Na*He2 and K*He2) exciplex formation for a small fraction of the alkali atoms. The lack of fluorescence quenching, which is present when the spectra are taken in bulk liquid helium, is due to the surface location of the alkali atoms on the helium droplets that allows the nascent Na*He and K*He exciplexes to desorb from the droplet and emit as isolated molecules. © 2000 American Institute of Physics.

Notes: We have monitored the time evolution of the fluorescence of K*He exciplexes formed on the surface of helium nanodroplets using reversed time-correlated single photon counting. In modeling the present data and that from our previous work on Na*He, we find that partial spin–orbit coupling as well as the extraction energy of helium atoms from the droplet contribute to the observed dynamics of both K*He and Na*He formation, which differ considerably after either D1(n 2P1/2←n 2S1/2) or D2(n 2P3/2←n 2S1/2) excitation for both K(n=4) and Na(n=3). Our quantitative prediction of the Na*He formation dynamics coupled with preliminary data on and modeling of the formation dynamics of K*He allow for extrapolation to the case of Rb*He. Spin–orbit considerations combined with a simple model of helium atom extraction from the matrix reveal the following predicted trend: as the choice of the alkali guest atom is moved down the periodic table, alkali atom–He exciplex formation along the 1 2Π3/2 surface occurs faster while formation along the 1 2Π1/2 surface occurs more slowly, ceasing to occur at all in the case of Rb. © 2000 American Institute of Physics.

Notes: 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.

Notes: 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.