State Population Distributions Research Articles

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Photodissociation dynamics of H2S at 121.6 nm and a determination of the potential energy function of SH(A 2Σ+)

A new and improved version of the technique of H atom photofragment translational spectroscopy has been applied to a study of H2S photodissociation at 121.6 nm. The primary fragmentation pathways leading to H+SH(A) fragments and H+H+S(1D) atoms are observed to dominate the product yield; the yield of H atoms formed in conjunction with ground state SH(X) fragments is undetectably small. The majority of the SH(A) fragments are formed in their v=0 level with a rotational state population distribution that spans all possible bound and quasibound rotational levels. The experimental determination of the energies of these hitherto unobserved high rotational states has enabled a refinement of the SH(A) potential energy function, an improved estimate of the SH(A) well depth (9280±600 cm−1), and thus of the SH(X) ground state bond dissociation energy D00 (S–H)=3.71±0.07 eV. All aspects of the observed energy disposal in the title photodissociation process may be understood, qualitatively, if it is assumed that (i) the primary fragmentations occur on the B̃ 1A1 potential energy surface and (ii) Flouquet’s ab initio calculations of portions of this surface [Chem. Phys. 13, 257 (1976)] correctly predict its gross topological features.

Fluorescence lifetime of the CO D′ 1Σ+ electronic state

The lifetime of the CO D′ 1Σ+ electronic state has been measured by laser induced fluoresence. A highly vibrationally excited CO(X) state population distribution was created by optical vibration–vibration pumping. The D′ population was created by a one photon UV laser excitation of CO(X,v″>30) and the fluorescence from D′–X was time resolved. The measured fluorescence lifetime of D′v′=2 is 3.87(30) ns.

Photodissociation dynamics of H2S(D2S) following excitation within its first absorption continuum

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of H2S(D2S) following excitation at a number of wavelengths in its first absorption continuum. The respective bond dissociation energies were determined as D0(H–SH)=3.90±0.03 eV and D0(D–SD)=4.00±0.02 eV. The measured vibrational energy disposal in the partner ground state SH(SD) fragments and the product angular distributions support and extend the earlier results of van Veen et al. [Chem. Phys. 74, 261 (1983)], while the deduced rotational and spin–orbit state population distributions in these fragments appear to be generally consistent with the recent measurements of Weiner et al. [J. Chem. Phys. 90, 1403 (1989)]. We present a schematic adiabatic potential energy surface for the dissociative 1A″ excited state via which it is possible to rationalize virtually all existing dynamical measurements relating to this fragmentation process.

Investigation of no scattering from organic monolayers: Spin-orbit state and vibrational state population distributions

The final internal energy distribution of NO molecules scattered from two different organic monolayers is investigated. Inelastic scattering of NO( 2Π 1 2 ) to give NO( 2Π 3 2 ) is found to be particularly efficient, the population in the upper NO( 2Π 3 2 ) state increasing with collision energy. A simple theoretical model is proposed to interpret these results and to explain their similarity to those observed in NO scattering from metallic surfaces. No vibrational excitation of the NO scattered from either of the two organic surfaces is observed, even though rotational excitation to channels well above the vibrational excitation threshold is seen.

The structure and dissociation dynamics of the Ne2Cl2 van der Waals complex

The structure and dynamics of Ne2Cl2 and Ne3Cl2 are studied by laser pump–probe spectroscopy. Analysis of a rotationally resolved B←X excitation band shows that Ne2Cl2 has a distorted tetrahedral structure with a Ne–Ne bond length of 3.23 Å and Ne2 center of mass to Cl2 center of mass distance of 3.12 Å. This structure is very close to that predicted by summing the atom–atom interactions. Excitation spectral shifts suggest a Ne3Cl2 structure with the neon atoms encircling the Cl2 bond axis. The total van der Waals binding energy of Ne2Cl2 is found to be between 145.6 and 148.6 cm−1, which is 20 cm−1 greater than 2*D0(Ne–Cl2)+D0(Ne2). For Cl2 stretching levels below υ′=10, transfer of one Cl2 vibrational quantum to the van der Waals vibrational modes is sufficient to dissociate both neon atoms from the complex. This indicates that the two neon atoms need not dissociate via independent, impulsive ‘‘half-collisions’’ which would require two Cl2 vibrational quanta. Observation of a NeCl2 dissociation fragment, however, indicates that such a sequential mechanism competes with the direct dissociation. Cl2 fragment rotational state population distributions for different initial vibrational levels are characterized using a simple rotational surprisal analysis. Comparison of these surprisal plots to those of the NeCl2 dissociation shows that as the size of the complex increases, so does the degree of statistical redistribution during the reaction. Even for Ne2Cl2, however, the extent of product rotational excitation is only weakly dependent upon the amount of energy available to the products and is always less than predicted by a statistical distribution between the translational and rotational product degrees of freedom.

Ultraviolet photodissociation dynamics of H2S and D2S

Nascent SH(X 2Πi, v″=0,1) and SD(X 2Πi, v″=0,1) rotational state population distributions, spin–orbit state population ratios, and Λ-doublet state population ratios have been measured following the UV excimer laser photodissociation of H2S (λ=193, 222, and 248 nm) and D2S (λ=193 and 222 nm), respectively. Nascent SH(X 2Πi, v″=0) rotational state distributions following 193 nm photodissociation of cold H2S in a free jet expansion vs 300 K H2S in a flowing gas cell were essentially the same, indicating that photofragment angular momentum must be originating predominantly in the dissociation event, and not from rotational energy contained in the parent triatom. Laser excitation spectra of SH(X 2Πi, v″=1) and SD(X 2Πi, v″=1) have been recorded for the first time. Rotational state distributions for SH(X 2Πi, v″) and SD(X 2Πi, v″) are independent of v″. Λ-doublet population ratios of the nascent photofragments are essentially unity for all the cases measured. The nascent rotational state distributions are consistent with an impact parameter model for the dissociating triatomic molecule.

Direct inelastic scattering of N2 from Ag(111). I. Rotational populations and alignment

The rotational state populations and the quadrupole and hexadecapole alignment moments of N2 scattered off clean Ag(111) are determined by resonance enhanced multiphoton ionization (REMPI). The scattered N2 is found to be highly aligned with its rotational angular momentum vector J parallel to the surface. The degree of alignment is found to increase with increasing rotational excitation. We see less than perfect alignment at intermediate J values indicating that the surface is not completely flat. The alignment is relatively insensitive to incident energy, incident angle, or surface temperature Ts. However, the rotational state population distributions show pronounced rainbows for higher incident energy and/or more grazing exit angle. The rotational state distributions are found to depend strongly on the final scattering angle at low Ts; this effect is markedly reduced at higher Ts. Time-of-flight measurements are used to determine the average velocity of the scattered N2 as a function of rotational level. It is found that higher rotational excitation correlates with lower average velocity and that the incident molecules lose 20%–30% of their translational energy to the solid. No correlation is found between velocity and alignment. A comparison is made with published results for the NO/Ag(111) system and a variety of theoretical models found in the literature.

State-selective detection of H 2 BY 1+1 REMPI via the C 1Π u (ν′=0, J′) states

It is shown that there are some advantages to determining ground state population distributions of H 2 by 1 + 1 REMPI using the C 1Π u (ν′=0) state as the intermediate state. The state distribution can be deduced more directly from the ion signal; in addition, a much improved sensitivity of <1.7×10 5 molecules/cm 3 per quantum state is achieved.

Spectroscopic diagnostics for a low-pressure plasma spray deposition system

Emission spectroscopy was utilised to characterise an arc-driven d.c. plasma jet used for low-pressure plasma spray deposition. Line of sight argon emission line intensities were measured in the wavelength range from 400 to 850 nm; nitrogen rotational spectra were measured from 300 to 450 nm. The Ar0 excitation temperature was calculated using the Boltzmann slope method, assuming partial local thermodynamic equilibrium (LTE). The degree of over-population of some energy states was also calculated. Data showed that partial LTE did not exist; therefore, calculated temperatures represent Ar electronic state population distribution. Temperatures ranged from 4220 to 7870 K and varied with arc power, chamber pressure and spatial position.

Energy Transfer and Ionization Processes in a Reduced-Pressure Argon ICP

The spectral characteristics of an argon/helium ICP operated at reduced-pressure are presented. The state population distributions deduced from quantitative intensity measurements for plasma neutral and cationic species suggest that the argon/helium plasma is essentially an argon plasma with respect to energy stratification. Comparisons are made with recently reported results for an atmospheric-pressure argon/helium ICP.

Studies of Energy Transfer and Ionization Processes in a Helium ICP

The spectral consequences of the introduction of varying concentrations of heavy rare gas atoms into a reduced-pressure helium ICP have been examined. A term-dependent quenching of He(I) emission was observed and is consistent with previous work on a kinetic model for this ICP. In addition, the ionization behavior of the added rare gases has been studied, and a simple rule seems to emerge from the results: the first ionization potential of helium represents an upper bound to the plasma energy available (probably through collisional processes) to excite and/or ionize added species. Although spectroscopic temperatures can be calculated from the state population distributions for the added neutral gas atoms, the fact that the values vary with concentration and with the chemical identity of the species betrays the fact that the plasma is not in local thermodynamic equilibrium with respect to neutral atoms. Quite differently, the results for plasma positive ions show the same statistically determined spectroscopic temperatures; thus, it appears that these ions are nearly equilibrated and also suprathermal in population distribution.

LASER-INDUCED FLUORESCENT STUDY OF NASCENT STATE DISTRIBUTION OF THE PRODUCTS FOR Ca+NF&lt;SUB&gt;3&lt;/SUB&gt;, CHF&lt;SUB&gt;3&lt;/SUB&gt; REACTIONS

The state population distributions of the nascent products for Ca+NF3 -> CaF+NF2 and Ca+CHF3 -> CaF+CHF2 reactions were studied by means of LIF in a beam-gas arrangement. From the LIF spectra of CaF and the computer simulations, the CaF vibrational distributions for both reactions were found to be similar to the Boltzmann distribution and the internal energy disposal of CaF was determined quantitatively. The results of the spectral simulation and the information theory computation indicate that NF2 and CHF2 radicals were vibrationally and rotationally excited. These tow reactions are thought to be direct collision reactions with the late exoeergicity on potential surface.

Reduction of 1+1 resonance enhanced MPI spectra to populations and alignment factors

A theory is presented to reduce 1+1 resonance enhanced multiphoton ionization (REMPI) spectra to accurate rovibrational state population distributions. Classical and quantum mechanical treatments are developed to model the polarization dependence of the REMPI signal from an initially aligned ground state having cylindrical symmetry. The theory includes the effects of saturation and intermediate state alignment. It is demonstrated that, for favorable cases, 1+1 REMPI allows the determination of the relative population as well as the quadrupole and hexadecapole moments of the alignment for rovibrational levels of a linear molecule. The classical treatment differs from that of the quantum treatment by less than 5% for rotational quantum numbers greater than J=4, suggesting that the classical treatment suffices for 1+1 REMPI in most molecular systems.

Reduction of 1+1 resonance enhanced MPI spectra to population distributions: Application to the NO A 2Σ+–X 2Π system

A two-step methodology is presented for extracting ground state population distributions and alignment factors from 1+1 resonance enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity as it is collected, generating an isopower spectrum. In the second step populations and alignments are derived from the isopower spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. This procedure is applied to a room temperature thermal distribution of nitric oxide using the 1+1 REMPI process in which lines of the NO A 2Σ+–X 2Π (0,0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions.

A method for the deconvolution of incompletely resolved CARS spectra in chemical dynamics experiments

We describe a method for deconvoluting incompletely resolved CARS spectra to obtain quantum state population distributions. No particular form for the rotational and vibrational state distribution is assumed, the population of each quantum state is treated as an independent quantity. This method of analysis differs from previously developed approaches for the deconvolution of CARS spectra, all of which assume that the population distribution is Boltzmann, and thus are limited to the analysis of CARS spectra taken under conditions of thermal equilibrium. The method of analysis reported here has been developed to deconvolute CARS spectra of photofragments and chemical reaction products obtained in chemical dynamics experiments under nonequilibrium conditions. The deconvolution procedure has been incorporated into a computer code. The application of that code to the deconvolution of CARS spectra obtained for samples at thermal equilibrium and not at thermal equilibrium is reported. The method is accurate and computationally efficient.

Laser induced excitation spectroscopy of copper trimer in various stages of supersonic expansion: Observation of fluorescence from dissociative levels

Gas phase spectroscopic studies of copper trimer are reported. Using laser induced excitation spectroscopy and a trimer source producing concentrations some three to six orders of magnitude in excess of those available from alternative devices, we obtain the visible spectrum of the trimer. This spectrum has been associated with a 2E″–2E′ transition of a D3h molecule in which both electronic states undergo Jahn–Teller distortion. Spectra obtained under a wide variety of experimental conditions encompass the wavelength range 6050–5180 Å corresponding to a range of effective temperatures. Data obtained in the present study are correlated with the recent studies of Morse et al. (laser vaporization, R2PI, and depletion spectroscopies) and Moskovits and co-workers (resonance Raman-matrix isolation). The observed spectroscopic features are in one-to-one correspondence with the detailed excited state vibronic calculations of Thompson et al. and in some respects may agree with the ‘‘first order’’ ground state vibronic calculations of Moskovits. The generated concentration of trimer molecules is such as to allow the characterization of strongly predissociative levels lost to the R2PI technique. The extent of the observed spectra allow the separation of level structure associated on the one hand with the excited state symmetric stretch mode and on the other with the vibronic levels resulting from the coupling of the asymmetric stretch and bending modes. Significant changes in those features which dominate observed excitation spectra as a function of effective temperature are interpreted in terms of a change in the ground state population distribution and hence an alteration in the configuration space connecting populated ground state and accessed excited state levels. These temperature dependent effects and observed hot band structure imply that the pseudorotation barrier in the ground electronic state of the trimer is considerably smaller than previously suggested.

Unimolecular reactions near threshold: The overtone vibration initiated decomposition of HOOH (5νOH)

Laser induced fluorescence detection of the fragments from the unimolecular dissociation of hydrogen peroxide initiated by direct excitation of the fourth overtone of the OH stretching transition (5νOH) is a means of measuring both vibrational overtone predissociation spectra and populations of individual quantum states of the OH products. Because the excitation adds insufficient energy to break the OO bond, the measurements detect reactions of molecules that initially possess substantial thermal vibrational–rotational energy (≥1100 cm−1). The spectroscopic data, in which hot band and high energy transitions are particularly prominent, are fit by a vibrational–torsional model that adiabatically separates the low frequency torsional vibration from the higher frequency OH and OO stretching vibrations. The spectroscopic analysis shows that the effective trans barrier in the torsional potential increases by 75 cm−1 for each quantum of OH stretching excitation but decreases by 50 cm−1 upon excitation of the OO stretching vibration. The product state population distributions demonstrate that the lowest rotational level is preferentially populated in the unimolecular decomposition. This result is inconsistent with phase space theory, which predicts substantially more rotational excitation, but agrees with the statistical adiabatic channel model. The predictions of the latter model agree with the data for HOOH (5νOH) as well as with previous results for HOOH (6νOH). The statistical adiabatic channel model contains one parameter in addition to the ones required in phase space theory, and the value of this parameter that provides the best agreement with the measurements is consistent with an independent analysis of thermal recombination data. The experiment serves to test statistical calculations critically because it explores reactions near the threshold energy, where the predicted product state distributions are most sensitive to the details of the theory.

Evaluation of 2-photon ionization spectroscope as a probe of NO ground state population distributions

We present a critical evaluation of the use of 2-photon ionization spectroscopy as a probe of NO ground state populations. The experimental simplifications which result from saturating the intermediate A 2Σ +← X 2Π transition are discussed, as are the criteria for establishing saturation. We also present experimental results which confirm the hypothesis that ionization is the rate limiting step in this 2-photon ionization process and discuss its application to trace impurity analysis in gases.

HIRIS rocketborne spectra of infrared fluorescence in the O3(ν3) band near 100 km

High‐resolution O3(ν3) spectral emission data, obtained between 80 and 125 km during an aurora by the rocketborne HIRIS cryogenic interferometer spectrometer, have been analyzed by a spectral simulation/least squares technique. Data obtained below 110 km exhibit behavior independent of local auroral strength and thus appear to represent quiescent nightglow. The derived vibrational state population distributions for levels υ3′ ≥ 1 can be expressed as vibrational “temperatures” of 400–600 K. This result is interpreted in terms of formation of O3(υ3′) by three‐body recombination of O with O2 as well as other radiative and collisional processes. Additional data, obtained near 120 km and sampling intense auroral arcs, indicate a hitherto unexpected auroral enhancement of O3(ν3) fluorescence intensity and vibrational state distribution.