Electronic quenching of OH A 2Sigma+ radicals in single collision events with molecular hydrogen: quantum state distribution of the OH X 2Pi products

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A 2Sigma+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X 2Pi population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A 2Sigma+ (nu'=0, N'=0), which has a significantly reduced fluorescence lifetime due to quenching by H2. The probe laser monitors the OH X 2Pi (nu", N") population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N") of the quenched OH X 2Pi products observed in nu"=1 and 2 as well as a pronounced propensity for quenching into the Pi(A') Lambda-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH A 2Sigma+ by H2 proceeds through conical intersections between two potentials of A' reflection symmetry (in planar geometry) that correlate with the electronically excited A 2Sigma+ and ground X 2Pi states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H2. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000)], these calculations reveal a steep gradient away from the OH-H2 conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH X 2Pi products.     

Electronic quenching of OH A 2Sigma+ radicals in single collision events with H2 and D2: a comprehensive quantum state distribution of the OH X 2Pi products

A pump-probe laser-induced fluorescence technique has been used to examine the nascent OH X (2)Pi product state distribution arising from non-reactive quenching of electronically excited OH A (2)Sigma(+) by molecular hydrogen and deuterium under single-collision conditions. The OH X (2)Pi products were detected in v'=0, 1 and 2; the distribution peaks in v'=0 and decreases monotonically with increasing vibrational excitation. In all vibrational levels probed, the OH X (2)Pi products are found to be highly rotationally excited, the distribution peaking at N'=15 when H(2) was used as the collision partner and N'=17 for D(2). A marked propensity for production of Pi(A') Lambda-doublet levels was observed, while both OH X (2)Pi spin-orbit manifolds were equally populated. These observations are interpreted as dynamical signatures of the nonadiabatic passage of the OH + H(2)/D(2) system through the seams of conical intersection that couple the excited state (2 (2)A') and ground state (1 (2)A') surfaces.     

Collisional quenching of OD A 2Σ+ by H2: experimental and theoretical studies of the state-resolved OD X 2Π product distribution and branching fraction

We report joint experimental and theoretical studies of outcomes resulting from the nonreactive quenching of electronically excited OD?A (2)Σ(+) by H(2). The experiments utilize a pump-probe technique to detect the OD?X (2)Π product state distribution under single collision conditions. The OD?X (2)Π products are observed primarily in their lowest vibrational state (v(") = 0) with substantially less population in v(") = 1. The OD?X (2)Π products are generated with a high degree of rotational excitation, peaking at N(") = 21 with an average rotational energy of 4600 cm(-1), and a strong propensity for populating the Π(A(')) Λ-doublet component indicative of alignment of the half-filled pπ orbital in the plane of OD rotation. Branching fraction measurements show that the nonreactive channel accounts for less than 20% of quenching outcomes. Complementary classical trajectory calculations of the postquenching dynamics are initiated from representative points along seams of conical intersections between the ground and excited-state potentials of OD(A (2)Σ(+),X (2)Π) + H(2). Diabatic modeling of the initial momenta in the dynamical calculations captures the key experimental trends: OD?X (2)Π products released primarily in their ground vibrational state with extensive rotational excitation and a branching ratio that strongly favors reactive quenching. The OD?A (2)Σ(+) + H(2) results are also compared with previous studies on the quenching of OH?A (2)Σ(+) + H(2); the two experimental studies show remarkably similar rotational energy distributions for the OH and OD?X (2)Π radical products.     

Reactive quenching of OH A (2)Σ(+) by O(2) and CO: Experimental and nonadiabatic theoretical studies of H- and O-atom product channels

The outcomes following collisional quenching of electronically excited OH A (2)Σ(+) by O(2) and CO are examined in a combined experimental and theoretical study. The atomic products from reactive quenching are probed using two-photon laser-induced fluorescence to obtain H-atom Doppler profiles, O ((3)P(J)) atom fine structure distributions, and the relative yields of these products with H(2), O(2), and CO collision partners. The corresponding H-atom translational energy distributions are extracted for the H + O(3) and H + CO(2) product channels, in the latter case revealing that most of the available energy is funneled into internal excitation of CO(2). The experimental product branching ratios show that the O-atom producing pathways are the dominant outcomes of quenching: the OH A (2)Σ(+) + O(2) → O + HO(2) channel accounts for 48(3)% of products and the OH A (2)Σ(+) + CO → O + HCO channel yields 76(5)% of products. In addition, quenching of OH A (2)Σ(+) by O(2) generates H + O(3) products [12(3)%] and returns OH to its ground X (2)Π electronic state [40(1)%; L. P. Dempsey, T. D. Sechler, C. Murray, and M. I. Lester, J. Phys. Chem. A 113, 6851 (2009)]. Quenching of OH A (2)Σ(+) by CO also yields H + CO(2) reaction products [26(5)%]; however, OH X (2)Π (v(") = 0,1) products from nonreactive quenching are not observed. Theoretical studies characterize the properties of energy minimized conical intersections in four regions of strong nonadiabatic coupling accessible from the OH A (2)Σ(+) + CO asymptote. Three of these regions have the O-side of OH pointing toward CO, which lead to atomic H and vibrationally excited CO(2) products and∕or nonreactive quenching. In the fourth region, energy minimized points are located on a seam of conical intersection from the OH A (2)Σ(+) + CO asymptote to an energy minimized crossing with an extended OH bond length and the H-side of OH pointing toward CO in a bent configuration. This region, exoergic with respect to the reaction asymptote, is likely to be the origin of the dominant O + HCO product channel.     

Rotational state-resolved reaction cross section in the reactions of state-selected CH with NO and with O2

A pure and highly intense state-selected pulsed supersonic CH(X (2)Pi) radical beam source was developed by use of the C((1)D)+H(2) reaction with the combination of the state selection and purification by an electrostatic hexapole field. Under the beam-cell condition, the elementary reactions of CH+NO and CH+O(2) were studied by using this state-selected CH beam. NH(A (3)Pi) [and NCO(A (2)Sigma(+))] formations and OH(A (2)Sigma(+)) formation were directly identified in the elementary reaction of CH+NO and CH+O(2), respectively. For the CH+NO reaction, the relative branching ratio sigma(NCO*)sigma(NH) of NCO(A (2)Sigma(+)) formation to NH(A (3)Pi) formation was determined to be 0.35+/-0.15. The state-selected reaction cross sections were determined for each rotational state of CH. In the CH+NO reaction, a remarkable rotational state dependence of the reactive cross section was revealed, while the CH+O(2) reaction showed little rotational state dependence.     

Vibrational distribution in NO(X2Pi) formed by self quenching of NO A 2Sigma+ (v=0)

Time-resolved FTIR has been used to study the emission from the NO X 2Pi (v) products formed both by fluorescence and by collisional self quenching of the NO A 2Sigma+ (v=0) state. Vibrational excitation has been observed in ground state NO with populations up to at least v=20. Under conditions where fluorescence is the dominant removal process the nascent distribution in ground state NO(v) was found to be determined by the relative magnitude of the emission coefficients. Collisional quenching by ground state NO populates higher vibrational levels in NO(v) than fluorescence. By comparing distributions acquired at different pressures and by using a surprisal analysis, a nascent distribution of NO(v=0-20) is estimated for collisional relaxation of NO A 2Sigma+ (v=0) by NO. This distribution was found to be slightly hotter than statistical (prior) and showed evidence of oscillations at specific vibrational levels. This work is one of the first to be published concerning the vibrational ground state products of the quenching of electronically excited molecules and the first to report emission over such a large number of vibrational levels.     

Quasiclassical trajectory study of the postquenching dynamics of OH A 2Σ+ by H2/D2 on a global potential energy surface

We report full-dimensional, electronically adiabatic potential energy surfaces (PESs) for the ground state (1A(')) and excited state (2A(')) of OH(3). The PESs are permutationally invariant fits to roughly 23,000 electronic energies (MRCI + Q/aVTZ). Classical trajectory calculations of the postquenching dynamics of OH A (2)Σ(+) are carried out on the 1A(') PES for H(2) and D(2), at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to "adiabatic" and "diabatic" models of the dynamics, respectively. Branching ratios of reactive to nonreactive channels from separate C(2v), C(∞v), and C(s) symmetries of CoIs are calculated, as are final rovibrational state distributions of OH and H(2) products. The rovibrational distributions of the OH and D(2) products, the D/H-atom translational energy distribution are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The vibrational state distributions of H(2)O and HOD (although not measured by experiment) are also presented.     

Ultrafast predissociation dynamics of water molecules excited to the electronic C and D states

Two-photon excitation with femtosecond laser pulses in the spectral range 240-250 nm was used to prepare vapor phase H(2)O and D(2)O in the C (1)B(1) and D (1)A(1) states. Both states are predissociated via the B (1)A(1) state, forming excited OH/OD(A (2)Sigma(+)) as well as ground state OH/OD(X (2)Pi). We used ultrashort infrared probe pulses (1.65-2.42 microm) to control the ratio between these excited and ground state fragments originating from the dissociation process. Time resolved detection of the OH/OD(A (2)Sigma(+)) --> OH/OD(X (2)Pi) fluorescence allows us to monitor the dynamics of the predissociation. For the heterogeneous predissociation out of the C(1)B(1) state life times of (0.5 +/- 0.1) ps and (1.2 +/- 0.1) ps were found for H(2)O and D(2)O, respectively. The purely homogeneous character of the predissociation out of the D (1)A(1) state was monitored.     

Nonadiabatic quantum reactive scattering of the OH(A 2Σ+) + D2

The seams of conical intersection exist between the ground (1 (2)A(')) and the first-excited (2 (2)A(')) electronic potential energy surfaces (PESs) of OH(A (2)Σ(+),X (2)Π) + H(2) system. This intersection induces the nonadiabatic quenching of OH(A (2)Σ(+)) by D(2). We present nonadiabatic quantum dynamics study for OH(A (2)Σ(+)) + D(2) on new five-dimensional coplanar PESs. The ab initio calculations of PESs are based on multireference configuration interaction (MRCI)/aug-cc-pVQZ level. A back-propagation neural network is utilized to fit the PESs and nonadiabatic coupling. High degrees of rotational excitation of quenched OH(X (2)Π) products are found in nonreactive quenching channel, and the quenched D(2) products are vibrationally excited up to quantum number v(2) (')=8. The theoretical results of nonadiabatic time-dependent wave-packet calculation are in good agreement with the existing experimental data.     

Channel switching effect in photodissociating N2O+ ion at 312.5 nm

A experimental observation is presented on the N2O+ photodissociation process, which exhibits a complete channel switching effect in a narrow energy range. The N2O+ ions, prepared at the X2Pi (000) state by (3+1) multiphoton ionization of neutral N2O molecules at 360.6 nm, were excited to different vibrational levels in the A2Sigma+ state in a wavelength range of 275-328 nm. Based on the estimates of total released kinetic energies from the time-of-flight mass spectrum, it was found that the dissociation pathway of N2O+ (A2Sigma+), NO+ (X1Sigma+) + N(4S) with lower dissociation limit, changes abruptly and completely to NO+ (X1Sigma+) + N(2D) with higher dissociation limit, in a excitation energy range of merely 250 cm(-1) at lambda approximately 312.5 nm. This phenomenon was explained by competition between the two dissociation pathways across the special excitation energy region.     

Photodissociation channels for N2O near 130 nm studied by product imaging

The photodissociation of N(2)O at wavelengths near 130 nm has been investigated by velocity-mapped product imaging. In all, five dissociation channels have been detected, leading to the following products: O((1)S)+N(2)(X (1)Sigma), N((2)D)+NO(X (2)Pi), N((2)P)+NO(X (2)Pi), O((3)P) + N(2)(A (3)Sigma(+) (u)), and O((3)P) + N(2)(B (3)Pi(g)). The most significant channel is to the products O((1)S) + N(2)(X(1)Sigma), with strong vibrational excitation in the N(2). The O((3)P) + N(2)(A,B):N((2)D,(2)P) + NO branching ratio is measured to be 1.4 +/- 0.5, while the N(2)(A) + O((3)P(J)):N(2)(B) + O((3)P(J)) branching ratio is determined to be 0.84+/-0.09. The spin-orbit distributions for the O((3)P(J)), N((2)P(J)), and N((2)D(J)) products were also determined. The angular distributions of the products are in qualitative agreement with excitation to the N(2)O(D (1)Sigma(+)) state, with participation as well by the (3)Pi(v) state.     

Slow electron velocity-map imaging spectroscopy of the C4H- and C4D- anions

High resolution photodetachment spectra of C4H- and C4D- obtained via slow electron velocity-map imaging (SEVI) are presented. The spectra reveal closely spaced transitions to the neutral 2Sigma+ and 2Pi states which can be distinguished based on the corresponding photoelectron angular distributions. The C4H ground state is confirmed as the X2Sigma+ state, with the excited A2Pi state lying only 213 cm(-1) higher (201 cm(-1) for C4D). The electron affinities (EAs) are slightly revised to EA (C4H)=28,497+/-8 cm(-1) and EA (C4D)=28,478+/-10 cm(-1). Progressions in low frequency bending vibrations are observed in both states, yielding experimental frequencies of nu7=179(169) cm(-1) and nu6=408(392) cm(-1) for the X2Sigma+ state of C4H (C4D), and nu7=220(215)cm(-1) and nu6=446(437) cm(-1) for the A2Pi state.     

Ground state potential energy curve and dissociation energy of MgH

New high-resolution visible emission spectra of the MgH molecule have been recorded with high signal-to-noise ratios using a Fourier transform spectrometer. Many bands of the A 2Pi-->X 2Sigma+ and B' 2Sigma+-->X 2Sigma+ electronic transitions of 24MgH were analyzed; the new data span the v' = 0-3 levels of the A 2Pi and B'2Sigma+ excited states and the v'=0-11 levels of the X 2Sigma+ ground electronic state. The vibration-rotation energy levels of the perturbed A 2Pi and B' 2Sigma+ states were fitted as individual term values, while those of the X 2Sigma+ ground state were fitted using the direct-potential-fit approach. A new analytic potential energy function that imposes the theoretically correct attractive potential at long-range, and a radial Hamiltonian that includes the spin-rotation interaction were employed, and a significantly improved value for the ground state dissociation energy of MgH was obtained. The v'=11 level of the X 2Sigma+ ground electronic state was found to be the highest bound vibrational level of 24MgH, lying only about 13 cm(-1) below the dissociation asymptote. The equilibrium dissociation energy for the X 2Sigma+ ground state of 24MgH has been determined to be De=11104.7+/-0.5 cm(-1) (1.37681+/-0.00006 eV), whereas the zero-point energy (v'=0) is 739.11+/-0.01 cm(-1). The zero-point dissociation energy is therefore D0=10365.6+/-0.5 cm(-1) (1.28517+/-0.00006 eV). The uncertainty in the new experimental dissociation energy of MgH is more than 2 orders of magnitude smaller than that for the best value available in the literature. MgH is now the only hydride molecule other than H2 itself for which all bound vibrational levels of the ground electronic state are observed experimentally and for which the dissociation energy is determined with subwavenumber accuracy.     

Photodissociation of vibrationally excited SH and SD radicals at 288 and 291 nm: the S(1D2) channel

Ultraviolet photodissociation of SH (X 2Pi, upsilon"=2-7) and SD (X 2Pi, upsilon"=3-7) has been studied at 288 and 291 nm, using the velocity map imaging technique to probe the angular and speed distributions of the S(1D2) products. Photodissociation cross sections for the A 2Sigma+<--X 2Pi(upsilon") and 2Delta<--X 2Pi(upsilon") transitions have been obtained by ab initio calculations at the CASSCF-MRSDCI/aug-cc-pV5Z level of theory. Both the experimental and theoretical results show that SH/SD photodissociation from X 2Pi (upsilon"相似文献   

A study of the reaction of N+ with O2: experimental quantification of NO+(a 3Sigma+) production (298-500 K) and computational study of the overall reaction pathways

The product branching ratios for NO+(X 1Sigma+) and NO+(a 3Sigma+) produced from the reaction of N+ with O2 have been measured at 298 and 500 K in a selected ion flow tube. Approximately 0.5% of the total products are in NO+(a) at both temperatures, despite the fact that the reaction to form NO+(a) is 0.3 eV exothermic. High-level ab initio calculations of the potential energy surfaces for the N+ + O2 reaction show that the reaction from N+(3P) + O2(3Sigma(g)) reactants starts with an efficient early stage charge transfer to the N(2D) + O2+(X 2Pi) channel, which gives rise to the O2+(X 2Pi) product and, at the same time, serves as the starting point for all of the reaction channels leading to NO+ and O+ products. Pathways to produce NO+(a 3Sigma+) are found to be less favorable than pathways leading to the major product NO+(X 1Sigma+). Production of N(2D) has implications for the concentration of NO in the mesosphere.     

Steering dissociation of Br2 molecules with two femtosecond pulses via wave packet interference

The dissociation dynamics of Br2 molecules induced by two femtosecond pump pulses are studied based on the calculation of time-dependent quantum wave packet. Perpendicular transition from X 1Sigma g+ to A 3Pi 1u+ and 1Pi 1u+ and parallel transition from X 1Sigma g+ to B 3Pi 0u+, involving two product channels Br (2P3/2)+Br (2P3/2) and Br (2P3/2)+Br* (2P1/2), respectively, are taken into account. Two pump pulses create dissociating wave packets interfering with each other. By varying laser parameters, the interference of dissociating wave packets can be controlled, and the dissociation probabilities of Br2 molecules on the three excited states can be changed to different degrees. The branching ratio of Br*/(Br+Br*) is calculated as a function of pulse delay time and phase difference.     

Investigation of spatially resolved spectra of OH and N2+ in N2 and H2O mixture wire-plate positive pulsed streamer discharge

Optical emission spectroscopy has been applied to study the spatially resolved measurements of the emission intensities of OH (A(2)Sigma-->X(2)Pi, 0-0) and N(2)(+) (B(2)Sigma(u)(+)-->X(2)Sigma(g)(+), 0-0, 391.4 nm) produced by a high-voltage positive pulsed streamer discharge consisting of a gas mixture of N(2) and H(2)O in a wire-plate reactor under severe electromagnetic interference at atmospheric pressure. The effects of pulse peak voltage, pulse repetition rate, and the added O(2) flow rate on the spatial distributions of the emission intensity of OH (A(2)Sigma-->X(2)Pi, 0-0) and N(2)(+) (B(2)Sigma(u)(+)-->X(2)Sigma(g)(+), 0-0, 391.4 nm) in the lengthwise direction (direction from wire to plate) are investigated. It has been found that the emission intensities of OH (A(2)Sigma-->X(2)Pi, 0-0) and N(2)(+) (B(2)Sigma(u)(+)-->X(2)Sigma(g)(+), 0-0, 391.4 nm) rise with an increase in both pulse peak voltage and pulse repetition rate and decrease with an increase in oxygen flows added in an N(2) and H(2)O gas mixture. The emission intensity of OH (A(2)Sigma-->X(2)Pi, 0-0) decreases with increasing the distance from the wire electrode. The emission intensity of N(2)(+) (B(2)Sigma(u)(+)-->X(2)Sigma(g)(+), 0-0, 391.4 nm) is nearly constant at 0-4mm from wire electrode, and sharply increases near the ground electrode. The vibrational temperature of N(2) (C) increases with increasing O(2) flows and keeps almost constant in the lengthwise direction under the present experimental conditions. The main physicochemical processes involved are also discussed in this paper.     

Fluorescence and metastability of N2O2+: theory and experiment

State-selective mass spectrometry has revealed one conclusive and another probable metastable state of the N2O2+ dication, assigned respectively as 1 3Pi at 38.5 eV and 2 3Pi at 42.5 eV. Photon coincidence experiments confirm that dissociation of 1 3Pi is preceded by a fluorescent transition to X 3Sigma- and also indicate that an identical mechanism occurs for 2 3Pi. Highly correlated MRCI calculations are performed at a range of N2O2+ geometries, from which both N-N and N-O bond stretching curves are generated. Substantial barriers along both coordinates are observed for 1 3Pi and 2 3Pi, although the increasing density of states at higher energy may allow spin-orbit or vibronic predissociation for 2 3Pi. Fragment emissions derived from N2O+ and N2O2+ are analyzed with the aid of glass filters, from which NO (X 2Pi<--A 2Sigma+) and vibrationally excited N2+ (X 2Sigmag+<--B 2Sigmau+) transitions are deduced.     

State-resolved dynamics of the CN(B2Sigma+) and CH(A2Delta) excited products resulting from the VUV photodissociation of CH3CN

Fourier transform visible spectroscopy, in conjunction with VUV photons produced by a synchrotron, is employed to investigate the photodissociation of CH3CN. Emission is observed from both the CN(B2Sigma+-X2Sigma+) and CH(A2Delta-X2Pi) transitions; only the former is observed in spectra recorded at 10.2 and 11.5 eV, whereas both are detected in the 16 eV spectrum. The rotational and vibrational temperatures of both the CN(B2Sigma+) and CH(A2Delta) radical products are derived using a combination of spectral simulations and Boltzmann plots. The CN(B2Sigma+) fragment displays a bimodal rotational distribution in all cases. Trot(CN(B2Sigma+)) ranges from 375 to 600 K at lower K' and from 1840 to 7700 K at higher K' depending on the photon energy used. Surprisal analyses indicate clear bimodal rotational distributions, suggesting CN(B2Sigma+) is formed via either linear or bent transition states, respectively, depending on the extent of rotational excitation in this fragment. CH(A2Delta) has a single rotational distribution when produced at 16 eV, which results in Trot(CH(A2Delta))=4895+/-140 K in v'=0 and 2590+/-110 K in v'=1. From thermodynamic calculations, it is evident that CH(A2Delta) is produced along with CN(X2Sigma+)+H2. These products can be formed by a two step mechanism (via excited CH3* and ground state CN(X2Sigma+)) or a process similar to the "roaming" atom mechanism; the data obtained here are insufficient to definitively conclude whether either pathway occurs. A comparison of the CH(A2Delta) and CN(B2Sigma+) rotational distributions produced by 16 eV photons allows the ratio between the two excited fragments at this energy to be determined. An expression that considers the rovibrational populations of both band systems results in a CH(A2Delta):CN(B2Sigma+) ratio of (1.2+/-0.1):1 at 16 eV, thereby indicating that production of CH(A2Delta) is significant at 16 eV.     

Temperature- and species-dependent quenching of NO A 2Sigma+(v'=0) probed by two-photon laser-induced fluorescence using a picosecond laser

We report improved measurements of the temperature-dependent cross sections for the quenching of fluorescence from the A 2Sigma+(v'=0) state of NO. Cross sections were measured for gas temperatures ranging from 294 to 1300 K for quenching by NO(X (2)Pi), H(2)O, CO(2), O(2), CO, N(2), and C(2)H(2). The A 2Sigma+(v'=0) state was populated via two-photon excitation with a picosecond laser at 454 nm, and the decay rate of the fluorescence originating from A 2Sigma+(v'=0) was measured directly. Thermally averaged quenching cross sections were determined from the dependence of the fluorescence decay rate on the quencher gas pressure. Our measurements are compared to previous measurements and models of the quenching cross sections, and new empirical fits to the data are presented. Our new cross-section data enable predictions in excellent agreement with prior measurements of the fluorescence lifetime in an atmospheric-pressure methane-air diffusion flame. The agreement resolves discrepancies between the lifetime measurements and predictions based on the previous quenching models, primarily through improved models for the quenching by H(2)O, CO(2), and O(2) at temperatures less than 1300 K.