H. L. Dai

Rotation-induced vibrational mixing in X̃ 1A1 formaldehyde: Non-negligible dynamical consequences of rotation

Individual rotation‐vibration levels of the formaldehyde X 1A1 state with 7400<Evib<8600 cm−1 have been examined by the stimulated emission pumping (SEP) technique. At low values of the rotational quantum number (J≤3), the SEP spectra were simple. The only vibrational levels which appeared in the spectra were those expected either to have large Franck–Condon overlap with the A 41 level or to have appreciable Fermi resonance with a nearby Franck–Condon allowed level. At higher J and Ka values, the spectra rapidly became more complex and the observed level densities at J≊10, Ka≊2 were several times larger than the known total density of vibrational levels. This increase in the density of spectrally accessible vibrational levels was a result of rotation‐induced mixing of the anharmonic vibrational basis functions (Coriolis coupling) which compromised the ‘‘goodness’’ of both vibrational and Ka quantum numbers. Coriolis matrix elements computed in a harmonic normal mode basis set qualitatively confirmed the...

Time‐resolved Fourier transform spectroscopy with 0.25 cm−1 spectral and <10−7 s time resolution in the visible region

The development of a new time‐resolved Fourier transform spectrometer that is capable of 0.25 cm−1 spectral resolution and better than 10−7 s temporal resolution in the visible is reported. The time‐resolved capability of the spectrometer is achieved by coupling a step‐scan interferometer to a transient digitizer/laser system. The operation of the spectrometer is described in detail, and scattered light and laser‐induced fluorescence spectra from an I2 gas cell are presented to demonstrate the temporal and spectral resolution of the spectrometer.

Fourier transform dispersed fluorescence spectroscopy: Observation of new vibrational levels in the 5000–8000 cm−1 region of ã 1A1 CH2

Dispersed fluorescence spectra from the CH2 b 1B1 state to highly excited vibrational levels of the a 1A1 state were recorded by a new technique, Fourier transform dispersed fluorescence spectroscopy. The spectra obtained clearly show the advantages of using a Fourier transform spectrometer for dispersing fluorescence, namely emission over a wide spectral range can be efficiently detected with high sensitivity and resolution. These advantages allow four new vibrational levels in the CH2 a 1A1 state to be observed; the (2,0,0) and (0,5,0) vibrational overtones and the (1,2,0) and (1,3,0) combination bands. The vibrational term values for these levels are given, along with the harmonic frequencies and anharmonicities for the v1 and v2 modes. From the (0,5,0) term value an improved estimate of the barrier height to linearity in the CH2 a 1A1 state is made.

Oxygen and pyridine on Ag(110) studied by second harmonic generation: Coexistence of two phases within monolayer pyridine coverage

Abstract The adsorption of oxygen and pyridine on Ag(110) in UHV is studied by second harmonic generation (SHG) from the surface. The adsorption of oxygen at room temperature (atomic) and at 110 K (molecular) causes a monotonic decrease in the second harmonic intensity with coverage which obeys simple Langmuir kinetics. Model fits to the data yield initial sticking coefficients, s0, of 0.0049 and 0.0088 for atomic and molecular oxygen adsorption, respectively. In contrast to oxygen adsorption, the changes in SHG signal induced by the exposure of Ag(110) to pyridine are more complicated and cannot be fit to a single-site Langmuir adsorption model. The SHG measurements indicate the presence of two distinct phases in the pyridine monolayer. A model, which depicts a broad transition from a flat-lying pyridine configuration at low coverages to a stand-up configuration at higher coverages, reproduced both the SHG and the work function measurements. The flat-lying phase dominates at low coverage, but the stand-up phase prevails when the coverage exceeds 0.3 monolayers. s0 for pyridine adsorption at 110 K was determined to be close to unity.

Observation of large vibration‐to‐vibration energy transfer collisions (ΔE≳3500 cm−1) in quenching of highly excited NO2 by CO2 and N2O

Time‐resolved Fourier transform infrared emission spectra, recorded after 475 nm excitation of NO2 in a CO2 or N2O bath, show IR emission from collisionally populated vibrational levels of the bath gas. The frequency of the observed bands proves that the emission arises from either the (1,00,1), (0,2l,1), and/or (0,00,2) levels of CO2 or N2O. From the pressure dependence of the emission intensity it was determined that these levels are populated by single collisions with excited NO2. Under typical conditions (1:10 ratio of NO2 to bath gas and 1–2 Torr total pressure) a steady state concentration is reached in our experiments where 0.016±0.006 multiply excited CO2 molecules, or 0.03±0.01 multiply excited N2O molecules were generated per laser excited NO2. A transition dipole coupling model is applied to explain these results, where the resonance conditions for vibration‐to‐vibration energy transfer are relaxed by extensive vibronic and vibrational couplings in highly excited NO2. In this model the energy‐d...

Transient vibrational spectroscopy by flash photolysis stimulated emission pumping: 3ν2 of singlet methylene

A vibration–rotation spectroscopic method based on stimulated emission pumping has been developed for transient molecules generated by flash photolysis. This method may be generally applicable to radicals with electronic excited states accessible by visible or UV laser pulses. This first demonstration on the bending v2 =3 level of a 1A1 CH2 generated a vibrational term value of 3950.45(7) cm−1 and the rotational constants A=25.665(40), B=11.470(7), and C=6.7140(61) cm−1. These constants indicate that the 3ν2 level is well below the bending‐potential barrier. Perturbations of several rotational levels by the triplet state are identified. In addition, and most interestingly, abnormally intense ΔKa =3 transitions were observed in the a←b 2163 band.

Coverage dependent phase transition of pyridine on Ag(110) observed by second harmonic generation

Second harmonic generation from a Ag(110) surface is shown to be sensitive to submonolayer converages of O2, O, pyridine, and benzene. The second harmonic intensity as a function of exposure indicates that the orientation and bonding of pyridine adsorbed at 110 K changes at about 0.3 L.

Stimulated emission spectroscopy of van der Waals vibrational levels of glyoxal(X̃ 1Ag)⋅Ar

Stimulated emission spectroscopy was used to study the electronic ground state vibration–rotation levels of van der Waals complexes generated in a supersonic beam. This technique can be used to study all the vibrational levels with nonvanishing Franck–Condon factors from the electronic excited states with 0.04 cm−1 resolution. Five van der Waals vibrational levels in the X state were directly observed in the stimulated emission spectra and were assigned to the fundamental and first overtone levels of the two Ar⋅glyoxal bending modes and the stretching fundamental level. In addition, the rotational constants and the structure were determined for both the A and X state complex, together with the vibrational term values and the widths of single rotational levels of the three glyoxal vibrations in the glyoxal⋅Ar complex, the C–C stretch v4=1, the CH wag v8=1 and the OCC bend v5=1 level.

A photoemitted electron‐impact ionization method for time‐of‐flight mass spectrometers

A simple and efficient electron‐impact ionization method for time‐of‐flight mass spectrometers is described. This method utilizes the photoelectrons emitted from the accelerating electrode surfaces upon UV laser irradiation as the electron source. Since no modification of the spectrometer is required, it provides a convenient way to perform electron‐impact ionization in a time‐of‐flight mass spectrometer originally designed for laser ionization. The detection sensitivity (∼109/cm3) and mass resolution (∼150) achieved by this method in our apparatus are sufficient for diagnostic purposes for cluster beam experiments. The observed ion intensities suggest that the photoelectron current density produced is comparable to those generated by conventional thermionic emission electron sources. This technique is generally applicable as an ionization method in time‐of‐flight mass spectrometers and is particularly useful when the available lasers are not suitable for photoionization detection due to wavelength or int...

Bending overtones and barrier height of ã 1A1 CH2 by flash photolysis stimulated emission pumping

The J≤7 and Ka≤3 rotational levels of the bending vibrational overtones, (0,2,0), (0,3,0), and (0,4,0), and the (1,1,0) combination band of a 1A1CH2 were characterized in detail by stimulated emission spectroscopy. The methylene was generated from ketene photolysis. Spectroscopic analyses using a rigid rotor Hamiltonian gave the vibrational term values and rotational constants. The latter were found to be strongly J and Ka dependent suggesting the inadequacy of the rigid rotor Hamiltonian for calculating the rotational energy to high accuracy. Effects of Coriolis, Fermi, and singlet–triplet perturbations were all observed in the spectra and discussed. Abnormally intense ΔKa=3 transitions were observed in the a←b, 2162 and 2163 bands. From fitting the experimentally measured bending vibrational term values by a model calculation based on the WKB theory, the barrier height to linearity in the a state was determined to be 9870 cm−1.