Specific Rotational Levels Research Articles

Molecular Beams Studies of the Dissociation of Highly Excited NO2 Induced by Molecular Colliders

NO2 in high vibrational levels was prepared in a pulsed molecular beam by laser excitation of the mixed 12A1/22B2 state to energies hν below dissociation threshold D0, D0− hν = 0−500 cm-1. The beam of excited molecules was crossed with pulsed, neat molecular beams of HCl, CO2, N2O, and NH3 at relative collision energies of ∼2000 cm-1, and the NO produced by collision-induced dissociation (CID) was detected state-selectively. The CID yield spectra obtained by monitoring specific NO rotational levels while scanning the excitation wavelength show spectral features identical with those in the fluorescence excitation spectrum of NO2. The yield of the CID products, however, decreases exponentially (compared with the fluorescence spectrum) with the increase of the amount of energy required to reach the threshold of appearance of the monitored NO state. The average energy transferred per activating collision with polyatomic colliders is in the range 130−200 cm-1, having values similar to or lower than those for diatomic and atomic colliders. This is in contrast to deactivating collisions, in which polyatomic colliders are in general more effective. The results are discussed in terms of a mechanism in which the NO2 molecules are activated by impulsive collisions creating a distribution of molecules in quantum states above D0 whose populations diminish exponentially with energy. The collisional activation is followed by unimolecular decomposition. The differences between the activation and deactivation pathways are rationalized in terms of the number of degrees of freedom available for energy transfer in each channel.

Rotational dependence of the quenching of electronically excited CH(A 2Δ) and CH (B 2Σ −) produced by laser photolysis of acetone at 193 nm

Electronic relaxation rate constants for specific rotational levels of CH(A 2Δ and B 2Σ −) have been measured. In contrast to similar measurements made earlier for other hydrides, the results obtained in the present work show an increase in the quenching efficiency upon rotational excitation. The electronically excited CH states are produced by multiphoton excitation and fragmentation of acetone upon photolysis at 193 nm by means of an ArF excimer laser light. The CG fragments are produced rotationally hot and the time evolution of rotational levels with N′⩽21 for CH(A 2Δ, v′=0) and N′⩽15 for CH(B 2Σ −, v′=0) can be measured.

Measurements of rotational energy transfer and quenching in OHA 2 ? +,?? = 0 at elevated temperature

Total rotational energy transfer rates have been measured at 1330 K for specific rotational levels in the OHA2σ+,υ′ = 0 state in collisions with H2O. Rotational levels ranging fromN′ = 0 to 15 were studied. Measurements were performed in the post-flame region of a stoichiometric H2/O2/He flame operating at 25 mbar. Quenching rates following excitation of individual upper rotational states were also measured. The RET and quenching rates both exhibit monotonic decreases with increasing rotational quantum number.

N(4S), N(2D), and N(2P) yields in predissociation of excited singlet states of N2

Predissociation in several singlet valence and Rydberg states of molecular nitrogen has been investigated using photofragment spectroscopy. We report here measurements of the yields of the atomic fragment products N(4S0), N(2D0), and N(2P0) from predissociation of specific rotational levels in the b′ 1Σu+ (v=9–13), c′ 1Σu+ (v=3,4), c 1Πu (v=3,4), and o 1Πu (v=3) states of N2. These states are prepared by laser excitation from the metastable a″ 1Σg+ (v=0) level in a fast (3 keV) molecular beam. Correlated atomic fragments from single molecular dissociation events are monitored using a position- and time-sensitive detector to obtain a complete and sensitive scheme for all possible N2 dissociation products. Dissociation of the N2 states is found to occur to N(2D0)+N(4S0), and N(2P0)+N(4S0) products; production of N(4S0)+N(4S0) is found not to occur from any of the states investigated here. Branching of the dissociation products between the two active limits is found to be strongly correlated with the energy of the predissociating level: levels lying <112 200 cm−1 above N2 X 1Σg+ (v=0) predissociate primarily to the N(2D0)+N(4S0) limit, while states above this energy primarily predissociate to N(2P0)+N(4S0). This behavior is discussed in terms of the potential energy structure of the triplet states of nitrogen that lead to predissociation.

J-selective Stark orientation of molecular rotation in a beam.

We propose a facile method to transform alignment of molecular rotation in a beam into transverse orientation with high efficiency via the second-order Stark effect in a homogeneous electric field E. The calculated J-orientation magnitude for a specific rotational level J exhibits a regular structure of equidistant dispersion-form signals in E 2 coordinates. Estimates performed for the NaK molecule show the possibility to produce rotational and isotopic selective J orientation for a fixed beam velocity v. The orientation appears in the direction perpendicular to the (v,E) plane, and the optimal angle between v and E is 45 o

Competition between multiphoton fragmentation channels in H2 and HD induced by intermediate states.

The proton and deuteron spectra following resonant multiphoton ionization of H 2 and HD at 193 nm through the E, F state have been studied at power densities of 10 10 W/cm 2 . Our results show that the intermediate state induces competition between 3(2+1)-photon ionization, dissociation, and dissociative ionization channels. We find that the competition depends on the specific vibrational and rotational levels excited in the intermediate state

High-frequency phase-modulated interferometer

In optically pumped submillimetre lasers, the laser molecule is excited from a rotational level of the ground vibrational state to a specific rotational level of an excited vibrational state. A population inversion can be achieved not only for the directly excited transition but also for rotational transitions immediately below this transition. With appropriate cavity adjustment, a formic acid laser has been operated simultaneously on three such transitions. When a non-linear detector is used, a low-frequency mixing product at approximately 1 MHz is produced. This phenomenon has been applied to produce phase modulation of a Mach-Zehnder interferometer. An accurate measurement of the refractive index of Mylar has been made with this instrument.

(2+1′) rotationally resolved resonance enhanced multiphoton ionization via the E 2∑+(4s,3d) and H 2∑+(3d,4s) Rydberg states of NO

The results of studies of ionic rotational branching ratios and photoelectron angular distributions resulting from (2+1′) resonance enhanced multiphoton ionization of NO via various high J (≊21.5) rotational branches of the E2∑+(4s,3d) and H2∑+(3d,4s) Rydberg states are presented. The rotational branching ratios show the expected ΔN=even rotational propensity rule with very small ΔN=odd signals. The branching ratios for the E2∑+ state are seen to be independent of photoelectron energy with the ΔN=+2 signals strongest and no appreciable higher rotational transfer peaks (‖ΔN‖≥3). The higher rotational transfer signal for ionization of the H2∑+ state are also negligible but the rotational branching ratios are strongly energy dependent due to a Cooper minimum in the l=3 partial wave of the kσ- and kπ-continua at a photoelectron kinetic energy of 2.6 eV and 2.9 eV, respectively. This leads to a strong rotational selectivity that can be exploited to produce ions in a specific rotational level. These consequences of Cooper minima close to threshold are quite general and their influence on rotational distributions should be readily observable in other molecular systems. The photoelectron angular distributions via both states show a strong energy dependence with a rapid change in the angular distributions around the Cooper minimum associated with the H2∑+ state.

An alternative mechanism for spin-forbidden photo-ionization of diatomic molecules and its rotation–electronic selection rules

The spin-forbidden photo-ionization of diatomic molecules is proposed. Spin orbit interaction is invoked resulting in the correction and mixing of the wave functions of different multiplicities. The rotation–electronic selection rules given, but not proven, by Dixit and McKoy for Hund's case a based on the conventional mechanism of electric dipole transition (see Chem. Phys. Lett. 128, 49 (1986) are rederived and expressed in a different format. This new format permits the generalization of the selection rules to other photo-ionization transitions caused by the magnetic dipole, the electric quadrupole, and the two- and three-photon operators. These selection rules, which are for transitions from one specific rotational level of a given Kronig reflection symmetry to another, will help understand rotational branching and the dynamics of interaction in the excited state. They will also help in the selective preparation of well-defined rovibronic states in resonant-enhanced multi-photon ionization processes.

Cooper minima and rotationally resolved resonance enhanced multiphoton ionization spectroscopy

We demonstrate that a Cooper minimum, close to threshold, in photoionization via an excited molecular Rydberg state can have a dramatic influence on the ionic rotational branching ratios. It is also shown that this behavior can be exploited to produce ions selectively in a specific rotational level. To illustrate this effect we present the results of ab initio calculations for (2+1′) resonance enhanced multiphoton ionization via the O11 (23.5) branch of the H 2Σ+(3d,4s) state of NO, where a Cooper minimum is found in l=3 of the kσ and kπ continua at photoelectron kinetic energies of 2.6 eV and 2.9 eV, respectively.

Collisional relaxation of H2CO (Ã 1A2, v4=1, J K a ,K c=132,12) by He, Ar, Xe, and N2

The collision-induced deactivation of H2CO in a specific rotational level in its first electronically excited singlet state, Ã 1A2, v4=1, JKa,Kc=132,12, has been investigated with a series of colliders, M=He, Ar, Xe, and N2. The target level was populated via the transition Ã←X̃, 410, 132,12←133,11 using a pulsed dye laser. The subsequent relaxation was monitored using the technique of transient gain spectroscopy (TGS) via the transition Ã→X̃, 201301414, 132,12→131,13, which coincides with the line of an Ar+ laser at λ=488.0 nm. The experiments yielded the overall collisional depopulation rate constants (velocity averaged cross sections) for He, Ar, Xe, and N2, respectively: (5.25±0.20)×10−10 cm3/molecule s (39 Å2), (5.22±0.18)×10−10 cm3/molecule s (86 Å2), (5.37±0.29)×10−10 cm3/molecule s (106 Å2), (8.25±0.29)×10−10 cm3/molecule s (125 Å2), where the uncertainties correspond to the 2σ standard deviations.

Infrared double-resonance lineshapes in strongly pumped spherical-top molecules

Strong double-resonance signals are observed in silane pumped with a TEA-CO 2 laser and probed with a tunable diode laser. The lineshapes of these signals have been modeled with an extension of three level density-matrix theory to the case of a large bandwidth pump laser. Up to 50% of the ground state population can be transferred to specific rotational levels of the dyad by CO 2 laser pumping. Collision-mediated two-photon excitation to triad levels has also been observed.

Collisional deactivation rates for A 2Σ + (ν′ = 1) state of OH

Rates for rotational transfer, vibrational crossover, and electronic quenching have been determined for specific rotational levels, N′, of A 2Σ +, ν′ = 1, OH radicals in the presence of N 2 and O 2 quenching gases. Measurements of total rates of depletion of specific rotational levels, rates of energy transfer out of the ν′ = 1 state, and fluorescence spectra were made at room temperature utilizing a low-pressure OH flow system. Total rates of depletion and rotational transfer rates for N 2 and O 2 quenching gases show a weak dependence upon N′. The N 2 quenching rates for these processes and for vibrational crossover were faster than the O 2 rates. Conversely, the O 2 rates for electronic quenching were greater than the N 2 rates.

Rotational state dependence of transient linewidths in the CO 2 00 01 vibrational level due to translational energy recoil from hot H and D atom collisions

The nascent transient linewidths of the CO 2 00 01, J→00 02, J−1 transitions were measured immediately following collisions with hot H and D atoms of 2.30 and 2.16 eV energy, respectively. All lineshapes were well fitted by Gaussian profiles, but were significantly broader than the room-temperature Doppler profiles. D atom collisions resulted in broader linewidths than H atom collisions. The observed widths are in good agreement with a simple billiard ball model for a specific rotational level J. However, the linewidth was observed to increase with increasing rotational state within the 00 01 manifold for both hot H and hot D atom collisions.

Rotational effects on the quenching of electronically excited CH

Electronically excited CH was produced by multiphoton dissociation of acetone at 248 nm. Measurements of the electronic relaxation of specific rotational levels of CH (A 2Δ) by acetone show that the quenching efficiency decreases as N′ is increased, in accordance with room temperature results on OH* and NH* and theoretical predictions.

Electronic quenching of highly rotationally excited OH(A 2Σ, ν′=0, 1) by H 2O

Population of specific high-lying rotational levels of OH (A 2Π) in ν′=0 and 1 is achieved by tunable dye laser excitation of the A←X transition of the OH product formed in O( 1D)+H 2O→2 OH(ν′=0, 1). Observation with a fast transient recorder/signal averager of the spontaneous A→X emission temporal profile as a function of H 2O pressure yields the rate constant, k H 2O , for collisional quenching following excitation of specific OH(A 2Π, ν′, N′, F′) states. This rate decreased with increasing rotational excitation in sensible agreement with a model in which long-range attractive forces on a highly directional potential surface dominate collisional quenching. Extrapolation of the observed emission lifetimes to P H 2O =0 yields optical lifetimes in good agreement with previously reported values.

The structure and bond energy of ArCl2

The geometry and bond energy of ArCl2 are obtained by the technique of parity selected excitation spectroscopy. This is a pump–probe technique in which the probe laser is set to detect specific rotational levels of the B state of Cl2 while the pump laser is tuned through an ArCl2 B–X band. Since the excitation spectrum obtained depends on the parity of the detected Cl2 rotational level, the ArCl2 complex must have a symmetrical, T-shaped geometry on average over the time scale of the experiment, ∼100 ps. Simulation of the observed spectra, which show partially resolved rotational structure, yields an argon to chlorine center of mass distance of 3.7 Å for both the X and B electronic states of the complex. By measuring the threshold for dissociation to the v=6, j=8 level of the Cl2 B state, the Ar–Cl2 bond energy was determined to be 178 cm−1 in the B state and 188 cm−1 in the X state. These results are quite different from those on the isovalent complex ArClF which is a linear molecule with an Ar–Cl bond length of 3.33 Å, and a bond energy of 230 cm−1.

Measurement and assignment of new FIR laser lines in12CH3OH and13CH3OH

We have found twelve new FIR laser lines in12CH3OH and thirty three in13CH3OH. Both molecules were pumped by a regular cw CO2 laser. We have also assigned 2013CH3OH laser lines to specific rotational energy levels in the excited C-O stretchhindered rotation combined states

State-to-state measurements of the vibrational predissociation dynamics of HeCl2.

Well-resolved state-to-state data are reported for the vibrational predissociation of specific rotational levels of ${\mathrm{HeCl}}_{2}$(B) with eight vibrational quanta in the Cl-Cl stretching mode. The state resolution is sufficient to clearly reveal the symmetry constraints on the dynamics as well as a node in the product-rotational-state distribution. ``Parity-selected'' excitation spectra are obtained which reduce spectral congestion and aid in the assignment of individual rovibronic transitions.

Spectroscopic investigation of the response of the NO 2-N 2O 4 chemical system to optical laser pumping

An argon laser beam was used to perturb the NO 2-N 2O 4 chemical system. The perturbation was monitored, in different experiments, with the laser-induced fluorescence and the IR optical double-resonance (IODR) techniques. The expected quantum perturbations which involve specific rotational levels in NO 2, X 2A 1 were observed. Unexpected chemical effects which involve the ground state concentration of both species and which are attributed to the response of the chemical equilibrium to the optical perturbation were also observed. The IODR results are discussed with the help of a model based on the Beer-Lambert law, which allows the role of the chemical response to be demonstrated.