Rotational State Populations Research Articles

Determination of the translational, rotational and vibrational degree of freedom for H 2 and D 2 desorbing from vanadium(1 0 0) + O

The rotational, vibrational and translational energy distribution for hydrogen (deuterium) molecules desorbing from a vanadium(1 0 0) + O surface have been determined using resonance enhanced multi-photon ionization time-of-flight spectroscopy. The translational energy has been found to be hyper-thermal for all selected internal states. However, the translational energy E kin for molecules in v=1 is smaller than for those in the vibrational state v=0. The influence of the rotational states J on the translational energy is more complex: for low J-states E kin increases with J, whereas for high J-states E kin decreases with increasing J. We have also measured the population of the individual rotational and vibrational states of the desorbing H 2 (D 2) molecules: In both cases the first excited vibrational state is overpopulated. The population of the J-states is hyper-thermal for low and high J but sub-thermal for medium J.

Characterisation of desorbing molecules by REMPI-TOF spectroscopy: the system H 2 (D 2)−V(1 0 0)+O

The internal state distribution (rotation, vibration) and the translational energy distribution of state selected molecules can be measured with the help of a REMPI (resonance enhanced multi-photon ionisation)-TOF (time-of-flight) spectrometer. We have performed measurements of this type on hydrogen and deuterium molecules desorbing from an oxygen covered V(1 0 0) surface. The rotational state population of the hydrogen and deuterium molecules in the vibrational ground state exhibits an overpopulation for J<3 and an underpopulation for 3< J<8. In addition we observe a slight overpopulation for J>9 for desorbing deuterium. The calculation of the mean rotational energy E rot leads in all cases to values significantly below the respective thermal equilibrium values (rotational cooling). For the vibrational state distribution we observe in all cases an overpopulation for molecules in the first excited vibrational state (vibrational heating). The translational energy distribution of desorbing deuterium strongly depends on the surface quality. On the flat, well annealed oxygen covered V(1 0 0) surface the mean translational energy of desorbing deuterium is hyperthermal, whereas the translational energy of molecules desorbing from a rough surface is closer to thermal.

The dynamics of formation of HCl products from the reaction of Cl atoms with methanol, ethanol, and dimethyl ether

The dynamics of ground state Cl(2P3/2) atom reactions with methanol, methanol-d1, ethanol, and dimethyl ether have been studied both experimentally and theoretically. The reactions were photoinitiated by 355 nm photolysis of Cl2 to produce monoenergetic Cl(2P3/2) atoms that react with ground electronic state organic molecules under single collision conditions. The rotational quantum state population distributions of the nascent HCl(ν′) products were probed by 2+1 resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. Nascent HCl(ν′=0) products from reaction of Cl atoms with methanol, methanol-d1 (CH3OD), ethanol, and dimethyl ether, at mean collision energies in the range of 5.6–6.7 kcal/mol, exhibit distributions of population over rotational levels that all peak at J′=3–5. The average rotational energies of the HCl(ν′=0) products for the respective reactions are 〈Erot〉=330±29, 300±24, 340±24, and 256±17 cm−1 (1σ uncertainties). Ab initio calculations were performed in order to examine the mechanisms of Cl atom abstraction of hydrogen from the alcohols and ether. Optimized geometrical structures and vibrational frequencies of molecular complexes and transition states on the reaction pathways were obtained at the MP2/6-311G(d,p) level and their energies were further refined at the G2 level of theory. Comparisons are drawn between the mechanisms and energetic pathways of the various reactions. The degree of rotational excitation of the HCl, which is significantly greater than for Cl atom abstraction of an H atom from alkanes, is attributed to a dipole–dipole interaction between the HCl and RCHOR′ (R, R′=H or CH3) moieties in the products’ region of the potential energy surface.

Excited state dynamics of Cl2O in the near ultraviolet

The state-resolved and isotope-specific detection of nascent ClO generated from the photodissociation of Cl2O parent molecules is performed by observing single-color (2+1) resonance enhanced multiphoton ionization (REMPI) spectra following excitation in the wavelength range from 336 to 344 nm; additionally state-resolved detection of nascent ClO is performed by observing single-color two photon laser-induced fluorescence. The REMPI spectrum is assigned to the ClO(C 2Σ−,v′=0←X 2ΠΩ,v=0) transition. The population of rotational states up to J=130 is evidence of large rotational excitation induced by a strong dependence of the excited potential energy surface (PES) on the Jacobi angle γ. Formation of the Π3/22 spin-orbit state is preferred: P(2Π1/2):P(2Π3/2)=0.30±0.05 suggesting significant radiationless transfer from the excited PES to closely lying neighboring states. The anisotropy parameter for the transition is determined to be β=0.35 independent of the ClO quantum state. The principal excited electronic state is assigned to a B21←A11 transition, in agreement with recent ab initio calculations. The measured β value is smaller than the limiting value of 1.0, suggesting dynamic reasons and simultaneous excitation to more than one PES. A fast and direct fragmentation process is deduced, where the upper limit for the lifetime of the excited dissociative state is calculated to be 300 fs.

Investigation of the state-to-state rotational relaxation rate constants for carbon monoxide (CO) using infrared double resonance

State-to-state rotational relaxation of carbon monoxide (CO) has been studied using an ir double resonance technique. Individual rotational lines of the (2-0) vibrational overtone band were pumped by a pulsed tunable ir laser and the subsequent evolution of the v=2 rotational population distribution was monitored by the absorption of a tunable cw ir laser via the (3-2) band transitions. Both the excitation and probe lasers were linearly polarized, with linewidths that were narrower than the CO Doppler width. Consequently, alignment and velocity relaxation effects were observed in these measurements. A data set consisting of 54 time-dependent rotational state population profiles was acquired. The full CO–CO rotational relaxation matrix, which consists of state-to-state rate constants for rotational levels up to J=29, was deduced from computer simulations of the data. Scaling and fitting laws were used to provide parametric representations of the rate constants. The three most common models, modified exponential gap, statistical polynomial-exponential gap (SPEG), and energy corrected sudden with exponential-power gap (ECS-EP) were investigated. We concluded that the SPEG law best reproduced the CO–CO rotational energy transfer data. A propensity to preserve the CO parity in rotational energy transfer was observed for collisions where the amount of energy transferred was small. Hence even ΔJ processes were favored for transitions between levels with low J values. For near-single collisions events a correlation was noted between the amount of rotational energy transferred and the degree of velocity distribution relaxation. This correlation yielded insights regarding the energy transfer dynamics.

Collisional dynamics of Bi2 A(0u+). II. State-to-state rotational energy transfer

Rotational-to-translational (R–T) energy transfer within v′=1 of the A(0u+) state of Bi2 has been investigated using spectrally resolved, laser induced fluorescence techniques. Spectrally resolved emissions from collisionally populated rotational levels of Bi2(A,v′=1) were observed for helium, neon, and argon collision partners after laser excitation of the high rotational levels J′=171, 201, and 231. Total rotational removal rates from the initially prepared state range from 2.8–8.9×10−10 cm3/molecule s. Collisional population of rotational states with |ΔJ|⩽56 was observed at pressures of 0.09–1.4 Torr. The state-to-state rates are adequately modeled by the energy based statistical power gap law.

Corrugation induced rotational excitation in photon/electron-induced desorption of ammonia: A three-dimensional quantum study

The photon/electron-stimulated desorption dynamics of ammonia from the Cu(111) and Cu(100) surfaces is investigated using a three-dimensional two-state quantum model, which includes the desorption and ammonia inversion coordinates as well as the “spinning” rotational degree of freedom. The desorption dynamics is followed by time-dependent wave packet on the ground or excited electronic state. Rotational state populations of both NH3 and ND3, obtained from the asymptotic wave packet, can be approximated by Boltzmann distributions and they show strong dependence on the symmetry and corrugation of the Cu surface. The NH3 rotational temperature of Trot=205 K on Cu(111) and 372 K on Cu(100) agree well with experimental observations. The desorbed NH3 is found to be rotational hotter than ND3, also in agreement with experiment. The addition of the rotational degree of freedom has little impact on other aspects of the dynamics, including desorption yield and vibrational excitation.

Extreme high rotational excitation of ClO

We observed the population of extremely high non-thermal rotational states of ClO (J max = 130.5) upon photodissociation of Cl 2 O. State-resolved and isotope-specific detection of nascent ClO is performed by observing single colour (2 + 1) resonance enhanced multiphoton ionisation (REMPI) spectra following excitation in the wavelength range from 336 to 344 nm. The REMPI spectrum is assigned to the ClO(C 2 Σ – , v′ = 0 ← X 2 Π Ω , v = 0) transition. The non-thermal population of rotational states is highly inverted, peaking at J = 107.5. The photodissociation of Cl 2 O is a classical example for the pure impulsive model, as a consequence of the Cl 2 O mass distribution and a weak dependence of the upper potential energy surface on the bond angle. Cl 2 O can thus be used as clean source for laboratory generation of highly excited ClO molecules with a defined non-thermal rotational population. In kinetic studies using ClO from Cl 2 O as precursor the high rotational excitation needs to be considered carefully.

Time dependence of rotational state populations of excited hydrogen moleculesin an RF excited plasma reactor

We report on time-dependent population distributions of excited rotational statesof hydrogen in a capacitively coupled RF discharge. The common model to obtainthe gas temperature from the rotational distribution is not applicable at alltimes during the discharge cycle due to the time dependence of the EEDF. Theapparent temperature within a cycle assumes values between 350 K and 450 K forthe discharge parameters of this experiment. We discuss the optimum time windowwithin the discharge cycle that yields the best approximation to the actualtemperature. Erroneous results can be obtained, in principle, with time-integratedmeasurements; we find, however, that in the present case the systematic erroramounts to only approximately 20 K. This is due to the fact that the dominantcontribution to the average intensity arises during that time window for whichthe assumptions underlying the analysis are best fulfilled. A similar analysiscan be performed for N+2 rotational bands with a small amount ofnitrogen added to the discharge gas. These populations do not exhibit the timevariations found in the case of H2.

Rotational distribution of the HCl products from the reaction of Cl( 2P) atoms with methanol

The rotational quantum state population distribution of the nascent HCl ( v ′=0) products of the reaction of Cl( 2P) atoms with methanol has been measured using photoinitiation of reaction and 2+1 resonance-enhanced multiphoton ionization detection of the products. At an average reagent collision energy of 5.6 kcal mol −1, the HCl ( v ′=0) is formed in a distribution of rotational states which peaks at J ′=3–5, and has a mean rotational energy of 〈E rot 〉=262±25 cm −1 (1 σ errors). The results are interpreted with reference to recent ab initio calculations of the energetics of the reaction pathway [J.T. Jodkowski et al., J. Phys. Chem. A 102 (1998) 9230], and comparisons are drawn with the much colder rotational distributions for the HCl products of reaction of Cl( 2P) atoms with alkanes.

Thermodynamic Studies in Subsonic Gas Flows Using a Laser Diode Raman Spectrometer

We present noninvasive, spatially resolved measurements on low-pressure, subsonic, laminar gas flows using Raman spectroscopy. From the Raman signal, the density and temperature of the flow can be extracted directly, with reasonable integration times (∼10 min-1 h). The use of a high-power laser diode makes it a very attractive low-cost, efficient technique. Temperatures studied range from 290 to 790 K; the spatial resolution was 2.5 × 1 mm2 in the flow cross section, and 10–20 μm in the flow direction, within a flow profile of diam. ∼ 15 mm. The flow was generated by a conical double-nozzle system with CO2 as the sample and Ar as the sheath gas. Vibrational and rotational state population and relaxation properties in the gas flow have been studied, and nonequilibrium conditions were found. A computational fluid dynamics (CFD) calculation is compared with the data; it supports the physical model suggested within the experimental errors.

Velocity map imaging and REMPI study of the photodissociation of CH 3SCH 3 from the first absorption band

The photodissociation of dimethyl sulfide (CH 3SCH 3) at 229 nm has been studied employing a combination of velocity map imaging and time-of-flight resonance-enhanced multiphoton ionization techniques to detect the CH 3 products. Translational energy and recoil angle distributions as well as rotational state populations have been determined for the CH 3 photofragments formed in the ground vibrational state. The electronic excitation of CH 3SCH 3 to the first absorption band is found to produce fast CH 3S+CH 3 ( v=0) recoiling products with a negative spatial anisotropy parameter of β=−0.85±0.05. The CH 3 ( v=0) products are rotationally cold, the rotational distribution peaking at N′′=3–4.

Temperature dependence of fast neutral–neutral reactions: a triatomic model study

Some neutral–neutral reactions are known to proceed rapidly at low temperatures as a consequence of strong inverse temperature power-law dependences of the rate constants. Previous calculations, based on capture approximations, failed to account for these experimental data. In this article, short-range effects (subsequent to capture) are investigated using a simple planar atom–diatom toy model based on pairwise atomic interactions. Reaction rate constants have been estimated in the temperature range of 25–300 K using a quasi-classical trajectory Monte Carlo approach. It is shown that a small short-range barrier in the entrance valley may significantly influence the reactivity. In particular, our crude triatomic model can reproduce a strong inverse temperature dependence of the rate constant in good agreement with experimental evidence for more complex systems. These predictions are interpreted using a 3-D representation of the effective potential surfaces, illustrating the crucial importance of vector correlations between partial angular momenta. Thus, the strong inverse power-law temperature dependence of the rate constant may be attributed to the population of higher rotational states of the reactants with increasing temperature.

Photofragmentation of ammonia at 193.3 nm: Bimodal rotational distributions and vibrational excitation of NH2(Ã)

Time-resolved Fourier transform infrared emission spectroscopy is used to measure the nascent rovibrational distribution of low-lying electronically excited NH2(Ã 2A1) produced in the 193.3 nm photolysis of room-temperature and jet-cooled ammonia. Emission is observed predominantly from NH2(Ã) states with rotational motion about the a-axis and without bending excitation, υ2′=0. A bimodal N′=Ka′ rotational state population distribution is observed with up to Ka′=7 in υ2′=0 and with maxima at Ka′=5 and Ka′=1. We suggest that the bimodal rotational distribution may result from the competition between planar and bent geometries during dissociation. Weaker emission from NH2(Ã) with bending excitation, υ2′=1 and 2, is detected; the υ2′=1, N′=Ka′ rotational state population distribution spans from Ka′=0 to the energetic limit of Ka′=4. The vibrational energy partitioning for the formation of NH2(Ã,υ2′=0):NH2(Ã,υ2′=1) is 3:1 and 2:1 in the room-temperature and jet-cooled conditions, respectively. An upper limit of the NH2(Ã,υ2′=2) population is ∼10% of the total NH2(Ã) photofragments. Emission from rotational states with N′&amp;gt;Ka′ (molecules with rotational excitation about the b/c-axes) is also observed. Under jet-cooled conditions the NH2(Ã) b/c-axes rotational temperature of ∼120 K is higher than that expected from the rotationally cold parent species and is attributed to a mapping of the zero-point bending motion in the ν4 H–N–H scissors bending coordinate of the NH3(Ã) predissociative state onto the NH2(Ã,υ2′,N′,Ka′)+H photofragments.

On the Interplay of Control Fields and Spontaneous Emission in Laser Cooling

Optimal control theory (OCT) is applied to the problem of cooling molecular rotations. The optimal field gives rise to a striking behavior, in which there is no noticeable increase in the lowest rotational state population until the last percent or so of the control interval, at which point the population jumps to 1. Further analysis of the intermediate time interval reveals that cooling is taking place all along, in the sense that the purity of the system, as measured by Tr(ρ2), is increasing monotonically in time. Once the system becomes almost completely pure, the external control field can transfer the amplitude to the lowest rotational state by a completely Hamiltonian manipulation. This mechanism is interesting because it suggests a possible way of accelerating cooling, by exploiting the cooling induced by spontaneous emission to all the ground electronic state levels, not just the lowest rotational level. However, it also raises a major paradox: it may be shown that external control fields, no mat...

Production of HCO from propenal photolyzed near 300 nm: Reaction mechanism and distribution of internal states of fragment HCO

The photodissociation dynamics of propenal in the near UV region is studied by detecting fluorescence of nascent fragment HCO in its transition B̃ 2A′−X̃ 2A′000. The yield is small and the wavelength range 300–287 nm of production is narrow. From the onset of yield of HCO versus the photolysis wavelength, the threshold of the formation of C2H3+HCO is determined to be 95.9±0.6 kcal/mol. At photolysis energy 34 032 cm−1 rotational states up to N=14 for K=0 and Ka=2 of HCO are populated. The fluorescence intensity is corrected for both the quantum yield of fluorescence and the effect of axis switching to yield the population of rotational states of HCO. The Ka=1 doublet states and the two spin states are about equally populated. The calculated distributions of HCO according to phase-space theory disagree with the experimental data indicating a nonstatistical distribution. Hence, radical products are expected to emanate from the triplet surface with a small exit barrier; this process competes with intersystem crossing back to the ground electronic state to generate molecular products C2H4+CO so causing a small yield of HCO and rapid depletion of excited propenal. This explains why the rate coefficient (&amp;gt;2×108 s−1) from the appearance curve of fragment HCO is greater than the calculated dissociation rate of triplet propenal.

Ab initio potential energy surface for the ground (2A′) state of H+SiO and rotationally inelastic collision cross sections for circumstellar H+SiO collisions

An ab initio potential energy surface has been calculated for the ground electronic state of the H+SiO system. The calculations were performed by first characterizing all the critical points of the system using ab initio complete active space-self consistent field (CAS-SCF) methods. These preliminary calculations were followed by multireference configuration interaction (CI) calculations covering a grid of 1748 nuclear geometries. The surface has been fitted to a global analytic form, using the recently proposed Reproducing Kernel Hilbert Space Method of Ho et al., [J. Chem. Phys. 104, 2584 (1996)] and preliminary vibrationally-rotationally inelastic scattering cross sections have been computed using infinite order sudden quantum scattering methods. The motivation of the research is the evaluation of these inelastic cross sections which are needed in the modeling of circumstellar SiO maser radiation. Significant structure has been observed in the computed inelastic cross sections. This structure is of the form which might be expected to give rise to the vibrational–rotational quantum state population inversions which in turn leads to the observed circumstellar maser action. An interesting feature of the computed potential energy surface is a line of maxima in nonlinear geometries. These maxima appear to arise from a nonsymmetry related conical intersection.

State-resolved collisional quenching of highly vibrationally excited pyridine by water: The role of strong electrostatic attraction in V→RT energy transfer

We report experiments that investigate the influence of long-range attractive forces on collisional energy loss from highly vibrationally excited molecules. State-resolved studies of energy transfer from highly vibrationally excited pyridine (μ=2.2 D) to water (μ=1.8 D) in a low-pressure environment at 298 K have been performed using high-resolution transient absorption spectroscopy of water at λ≈2.7 μm. Pyridine in its ground electronic state with 37 900 cm−1 of vibrational energy was prepared by absorption of pulsed ultraviolet light (λ=266 nm) to the S1 state, followed by rapid internal conversion to the S0 state. Collisions between vibrationally excited pyridine and water that result in rotational and translational excitation of the ground vibrationless state of H2O (000) were investigated by monitoring the populations of individual rotational states of H2O (000) at short times following pyridine excitation. The infrared probe of water was the highly allowed asymmetric stretching (000→001) transition. The nascent distribution of rotationally excited H2O (000) states is well described by a thermal distribution with a rotational temperature of Trot=770±80 K. Doppler-broadened transient linewidth measurements yield the velocity distributions of the recoiling H2O (000) molecules that correspond to center-of-mass translational temperatures of Ttrans∼515 K for all water rotational states investigated. Additionally, rate constants for energy gain in individual water states were determined, yielding an integrated rate constant of k2int=1.1×10−11 cm3 mol−1 s−1 for the appearance of H2O (000) with Erot=1000–2000 cm−1. These results are compared with previous relaxation studies of excited pyrazine (μ=0 D) with water and of excited pyridine with CO2 (μ=0 D), and the influence of electrostatic attraction on the relaxation dynamics is discussed.

Electron-impact rotational excitation of CH+

New coupled-state R-matrix calculations are performed at energies up to 1 eV to give rotational excitation and de-excitation cross-sections for electron collisions with CH+. Rotational excitations with Delta j up to 7 are considered. Transitions with Delta j up to 6 are found to have appreciable cross-sections, those with Delta j = 2 being comparable to (indeed slightly larger than) those with Delta j = 1, the only ones considered previously. Rates for electron temperatures up to 15 000K and critical electron densities are given. A model of CH+ rotational populations shows them to be relatively insensitive to electron temperatures in the range 800-2000K, but to depend strongly upon electron density. The models predict a significant population in rotational states with j > 1 even at low electron densities; at higher densities the possibility exists that electron collisions alone might explain the recently observed populations in the planetary nebula NGC 7027.

Rotational relaxation of CO in He, Ar, N 2 and CO free jets

We report terminal rotational state distributions for CO in free jets employing buffer gases of helium, argon, nitrogen, and carbon monoxide. These distributions are obtained from 2+2 resonance enhanced multiphoton ionization (REMPI) spectra of the CO A 1 Π state. All of the distributions show significant non-equilibrium character having excess population which increases as J increases. In He and CO buffers, for which CO translational temperature information is available, the lowest J CO rotational states appear to be in equilibrium with the CO parallel translation. It is suggested that rotational state populations in high moment of inertia molecules provide a convenient means to probe translational temperatures in free jet expansions.