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Photofragmentation of mass-selected aniline+(water)n (An+Wn, n=4–20) clusters is investigated over photon energies ranging from 1.65 to 4.66 eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An+Wn clusters and binding energies. The authors find that the An+W6 cluster has a highly symmetric structure and its binding energy in An+W6→An+W5+W stands out. This is in line with the experimental results showing that n=6 is a magic number in the mass distribution and An+W6 is relatively stable in metastable decay. [ABSTRACT FROM AUTHOR]

Distinct hydrogen bonding patterns are predicted in the amide I and amide A vibrational bands of four dominant conformations of antamanide using anharmonic vibrational Hamiltonians constructed at the DFT and AM1 levels. We show how these conformations may be distinguished using coherent three pulse infrared measurements with several pulse polarization configurations in the amide I region. The amide A hydrogen bonded N-H stretching modes are highly localized and have conformation-dependent frequencies, but their anharmonicities are insensitive to local structure at the hydrogen bond distances in antamanide. [ABSTRACT FROM AUTHOR]

Vibrational exciton Hamiltonians for the amide I and amide A modes of both the α- and 3[sub 10]-helical conformations of a fifteen unit polyalanine oligomer CH[sub 3]-CO(Ala)[sub 15]-NHCH[sub 3] are constructed using density-functional calculations for smaller model peptides. Energy levels as well as the transition dipoles of all singly and doubly excited-state manifolds are calculated. A variety of C[sub 13]-substituted isotopic derivatives are examined with respect to their ability to reveal differences in local secondary structures in two-dimensional infrared spectra in the amide I region. Amide mode anharmonicities are predicted to be valid indicators of secondary helical structures. [ABSTRACT FROM AUTHOR]

Mass spectral distributions and time-of-flight spectra of C[sup +][sub n] (n = 1-24) ions escaping from a carbon plasma plume generated by laser (λ = 1064 nm) ablation of a graphite target are analyzed to shed light on their formation mechanisms, expansion dynamics, and laser-plume interaction. In particular, by shining the whole or a slice of the plume with a photodissociation laser (λ=266 nm) delayed with respect to the ablation laser, we examine the effects of the laser-plume interaction on the mass distribution as well as photodissociation channels of carbon-cluster cations. Also, we present experimental data that show a clear linear dependence of the velocities of C[sup +][sub n] ions on 1/&Mradic;. This result confirms that C[sup +][sub n] ions are mainly accelerated by a transient local electric field in the plume. [ABSTRACT FROM AUTHOR]

We have used the method of Shapiro, Hepburn, and Brumer to control the ionization rates of HCl and CO. This method is based on the principle of quantum mechanical interference between two competing paths. In this case the two paths are one- and three-photon excitation of an intermediate Rydberg state. A vacuum ultraviolet (VUV) (110–115 nm) laser beam was produced by third-harmonic generation from an UV beam in Kr gas. The relative phases of the two beams were controlled by passing them through a chamber containing either Ar or H2. The laser beams were focused into a molecular beam, and the ionization products were measured in a time-of-flight mass spectrometer. As the pressure of the phase-tuning gas was varied, the ionization signal was found to oscillate with a period that is proportional to the difference of the indices of refraction of the tuning gas at the UV and VUV wavelengths. We have applied this technique to transitions involving different intermediate electronic states [the j 3Σ-(0+), H 1Σ+(0+), and m 3Π(1) states of HCl and the B 1Σ+ state of CO], different rotational lines of the same electronic transition, and different competing processes (ionization vs dissociation) following excitation of the intermediate state. [ABSTRACT FROM AUTHOR]

We have used the method of coherent phase modulation, first proposed by Brumer and Shapiro for controlling the rates of chemical reactions, to measure the refractive index of a gas in the vacuum ultraviolet. In these experiments we populated a Rydberg state of HCl or CO by simultaneous absorption of one VUV photon and three UV photons. Molecular ions were generated by absorption of an additional UV photon and detected with a time-of-flight mass spectrometer. The phase difference for the UV and VUV beams was altered by passing them through a cell containing either H2 or Ar. By varying the pressure in the cell we could control this phase difference and thereby modulate the ion signal. From the modulation frequency of the signal it was possible to determine the difference between the refractive indices of the gas at the UV and VUV wavelengths. Using reliable refractive indices in the UV, we extracted from our data values for the VUV indices. These values are in quantitative agreement with a calculation that uses dipole oscillator strengths constructed using constrained oscillator strength methods, taking into account the dispersion resulting from individual rotational transitions. The experimental method can be used very close to an optical transition of the refractive medium, provided that the refractive index does not vary appreciably across the bandwidth of the VUV laser. [ABSTRACT FROM AUTHOR]

Coherent control of the ionization of HCl was achieved by simulataneous 3+1 and 1+1 multiphoton excitation in a molecular beam machine, using either three 336 nm photons or one 112 nm photon to resonantly excite the intermediate j 3Σ-(Ω=0+) state. The phase difference between the two laser beams was continuously varied by passing the radiation through a cell containing either Ar or H2 gas. [ABSTRACT FROM AUTHOR]

Photofragmentation of mass-selected aniline(+)(water)n (An(+)Wn, n=4-20) clusters is investigated over photon energies ranging from 1.65 to 4.66 eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An(+)Wn clusters and binding energies. The authors find that the An(+)W6 cluster has a highly symmetric structure and its binding energy in An(+)W6-->An(+)W5+W stands out. This is in line with the experimental results showing that n=6 is a magic number in the mass distribution and An(+)W6 is relatively stable in metastable decay.