Nascent Rotational State Distributions Research Articles

Rotational and Vibrational State Distributions of CsH and Relative Reactivity in Reactions of Cs(62D, 72D) with H2

By using a pump-probe technique, the nascent rotational and vibrational state distributions of CsH are obtained in the Cs(62D,72D) plus H2 reaction. The nascent CsH molecules are found to populate the lowest two vibrational (ν″ = 0 and 1) levels of the ground electronic state. By comparing the spectral intensities of the CsH action spectra with those of pertinent Cs atomic fluorescence excitation spectra, the relative reactivity with H2 is in an order of 62D3/2 > 62D5/2 > 72D3/2 > 72D5/2. The rotational temperatures are found to be slightly below the cell temperature. The relative fractions (⟨fV⟩, ⟨fR⟩, ⟨fT⟩) of average energy disposal are derived as (0.2,0.12,0.68), (0.2,0.12,0.68), (0.07,0.04,0.89) and (0.07,0.04,0.89) for the 62D3/2, 62D5/2, 72D3/2 and 72D5/2, respectively. The major available energy is released as translation. These results support that the reaction mechanism of Cs(62D,72D) plus H2 is primarily a collinear abstraction and not an insertion.

Exact state-to-state quantum dynamics of the F+HD→HF(v′=2)+D reaction on model potential energy surfaces

In this paper, we present the results of a theoretical investigation on the dynamics of the title reaction at collision energies below 1.2 kcal/mol using rigorous quantum reactive scattering calculations. Vibrationally resolved integral and differential cross sections, as well as product rotational distributions, have been calculated using two electronically adiabatic potential energy surfaces, developed by us on the basis of semiempirical modifications of the entrance channel. In particular, we focus our attention on the role of the exothermicity and of the exit channel region of the interaction on the experimental observables. From the comparison between the theoretical results, insight about the main mechanisms governing the reaction is extracted, especially regarding the bimodal structure of the HF(v = 2) nascent rotational state distributions. A good overall agreement with molecular beam scattering experiments has been obtained.

Rotational and vibrational state distributions of NaH in the reactions of Na(4S2,3D2,and6S2) with H2: Insertion versus harpoon-type mechanisms

By using a pump-probe technique, the nascent rotational and vibrational state distributions of NaH are obtained in the Na(4 (2)S,3 (2)D, and 6 (2)S) plus H(2) reactions. The rotational distributions for the Na(4 (2)S,3 (2)D) reactions yield a bimodal feature with a major component peaking at J=20-22, similar to that obtained previously in the 4 (2)P reaction, whereas the Na(6 (2)S) reaction gives rise to a distinct distribution with a much lower rotational temperature. The vibrational populations (v=0-4) for these 4 (2)S, 3 (2)D, and 6 (2)S reactions are characterized by corresponding temperatures of 1692+/-120, 819+/-35, and 5329+/-350 K. Due to a significant contribution of configurational mixing between different states with the same symmetry, the collision species initiated from the 4 (2)S and 3 (2)D states are anticipated to track along the entrance surface in a near C(2v) symmetry, then undergo nonadiabatic transition to the inner limb of the reactive 2A(') surface. In contrast, the reaction pathway for the Na(6 (2)S) state with a significantly reduced ionization energy is anticipated to follow a harpoon-type mechanism via a (near) collinear configuration. The increased atomic size of Na may hinder the insertion approach.

Ab initio investigations of the radical-radical reaction of O(P3)+C3H3

We present ab initio calculations of the reaction of ground-state atomic oxygen [O(P3)] with a propargyl (C3H3) radical based on the application of the density-functional method and the complete basis-set model. It has been predicted that the barrierless addition of O(P3) to C3H3 on the lowest doublet potential-energy surface produces several energy-rich intermediates, which undergo subsequent isomerization and decomposition steps to generate various exothermic reaction products: C2H3+CO, C3H2O+H, C3H2+OH, C2H2+CHO, C2H2O+CH, C2HO+CH2, and CH2O+C2H. The respective reaction pathways are examined extensively with the aid of statistical Rice-Ramsperger-Kassel-Marcus calculations, suggesting that the primary reaction channel is the formation of propynal (CHCCHO)+H. For the minor C3H2+OH channel, which has been reported in recent gas-phase crossed-beam experiments [H. Lee et al., J. Chem. Phys. 119, 9337 (2003); 120, 2215 (2004)], a comparison on the basis of prior statistical calculations is made with the nascent rotational state distributions of the OH products to elucidate the mechanistic and dynamic characteristics at the molecular level.

The Sc+NO→ScO+N reaction: Rotational state distribution in ScOX 2Σ+(v″=0)

The Sc+NO→ScO+N reaction has been investigated in a beam-gas arrangement, with characterization of ScO products by cw laser-induced fluorescence: absorption versus laser frequency over the A 2Π(v′=1)–X 2Σ+(v″=0) band and fluorescence over the A 2Π(v′=1)–X 2Σ+(v″=1) one. It leads to the direct determination of the nascent rotational state distribution in the X 2Σ+(v″=0) level of ScO. This distribution is close to a Prior statistical one, with a well-characterized weak “surprisal,” indicating that a momentum constraint takes place during the reaction process. In the frame of this statistical distribution, a new accurate value for the dissociation energy of ScO is proposed: D00(ScO)=(6.92±0.01) eV. Spectroscopic data are reported for the A 2Π(v′=1)–X 2Σ+(v=0) band, up to N=98.

Energy-dependent cross sections and nonadiabatic reaction dynamics in F(2P3/2,2P1/2)+n–H2→HF(v,J)+H

High-sensitivity direct IR laser absorption methods are exploited to investigate quantum state-resolved reactive scattering dynamics of F+n-H2(j=0,1)→HF(v,J)+H in low-density crossed supersonic jets under single collision conditions. Nascent rotational state distributions and relative cross sections for reactive scattering into the energetically highest HF (v=3,J) vibrational manifold are obtained as a function of center-of-mass collision energies from Ecom=2.4 kcal/mole down to 0.3 kcal/mole. This energy range extends substantially below the theoretically predicted transition state barrier [Ebarrier≈1.9 kcal/mole; K. Stark and H. Werner, J. Chem. Phys. 104, 6515 (1996)] for the lowest adiabatic F(2P3/2)+H2 potential energy surface, therefore preferentially enhancing nonadiabatic channels due to spin–orbit excited F*(2P1/2) (ΔEspin–orbit=1.15 kcal/mole) in the discharge source. The HF (v=3,J) cross sections decrease gradually from 2.4 kcal/mole down to the lowest energies investigated (Ecom≈0.3 kcal/mole), in contrast with exact adiabatic quantum calculations that predict a rapid decrease below Ecom≈1.9 kcal/mole and vanishing reaction probability by Ecom≈0.7 kcal/mol. Further evidence for a nonadiabatic F*(2P1/2) reaction channel is provided by nascent rotational state distributions in HF (v=3,J), which are >2–3-fold hotter than predicted by purely adiabatic calculations. Most dramatically, the nascent product distributions reveal multiple HF (v=3,J) rovibrational states that would be energetically inaccessible from ground state F(2P3/2) atom reactions. These quantum state resolved reactive scattering studies provide the first evidence for finite nonadiabatic dynamics involving multiple potential energy surfaces in this well-studied “benchmark” F+H2 reaction system.

Dissociative excitation of HCOOH by single-vacuum ultraviolet and two-ultraviolet photon

Dissociative excitation processes of HCOOH in the vacuum ultraviolet (VUV) region were studied by single-VUV photon with synchrotron radiation source and by two-ultraviolet (UV) photon with KrF excimer laser. In the VUV dissociation, fluorescence excitation cross sections for the OH(A) and HCOO* were separately determined in the 106–155 nm region. The branching fraction was found to be a function of the VUV excitation wavelength. The magnitude is σOH(A)/[σOH(A)+σHCOO*]=0.13 at 124.5 nm and gradually increases to 0.39 at 110 nm. In the UV multiphoton dissociation at 249 nm, OH(A) and HCOO* fragments were also identified by a fluorescence spectrum. The production of OH(A) was shown to take place in the two-UV photon absorption of HCOOH. Nascent rotational and vibrational (V/R) state distributions of OH(A 2Σ+) produced via the photodissociation at a single excitation energy of 9.96 eV (124.5×1/249 nm×2), HCOOH+nhν(n=1,2)→HCO+OH(A 2Σ+), were determined by simulation analysis of the dispersed fluorescence spectra. The internal state distributions were found to be of the relaxed type, and rotational distribution could be approximated by a Boltzmann distribution. One-VUV photon excitation gave the best-fit rotational temperature Tr(v′=0)=3000 K and vibrational population ratio Nv′=1/Nv′=0=0.14, while two-UV photon excitation showed Tr(v′=0)=2000 K with Nv′=1/Nv′=0=0.12. Possible mechanisms for the OH(A) formation by both excitation sources were examined based on simple theoretical models. The degree of internal excitation is not consistent with a direct dissociation on a repulsive surface, and neither is a dissociation from a long-lived intermediate state. The formation of OH(A 2Σ+) is interpreted as dissociation of an electronically excited intermediate state, leading to the formation of OH(A)+CHO, populated competitively via an electronic predissociation process. The substantially different V/R distributions observed are dependent on the excited precursor state initially accessed, and may result from the constraint in the competing predissociation step that follows.

Reactions of C( [formula omitted]) with H 2, HD and D 2: kinetic isotope effect and the CD/CH branching ratio

Electronically excited carbon atom C( 1 D ) was produced by the 308 nm two-photon dissociation of C 3O 2. CH and CD radicals produced by the reactions of C( 1 D )+H 2, HD and D 2 were probed by laser-induced fluorescence. The nascent rotational state distributions of CH( v=0) and CD( v=0) were measured under single collision conditions. The distributions measured in the present study were close to those obtained in the previous studies using the 157 nm photodissociation. The rate constants at the room temperature were measured to be (2.0±0.6)×10 −10, (1.7±0.4)×10 −10 and (1.4±0.3)×10 −10 cm 3 molecule −1 s −1 for the reactions with H 2, HD and D 2, respectively. The CD/CH branching ratio in the C( 1 D )+HD reaction was determined to be 1.6±0.1. The preference of the CD production suggests that this reaction proceeds mainly via the HCD intermediate complex.

The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide: Excitation of the infrared inactive (100), (020), and (0220) bath vibrational modes

The collisional quenching of highly vibrationally excited pyrazine, C4H4N2, by CO2 has been investigated using high resolution infrared transient absorption spectroscopy at a series of cell temperatures. Attention is focused on collisions which result in excitation of the Fermi-mixed bath vibrational states (1000) and (0200), along with the unmixed overtone bend state (0220). The vibrationally hot (Evib≈5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the CO2 molecules in each vibrationally excited state are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions and velocity recoils, strongly suggest that the vibrational excitation of CO2 is dominated by a long-range electrostatic interaction despite the fact that the dipole transition matrix elements connecting the CO2 ground state to the excited states vanish for the isolated molecule. The vibrational energy transfer is accompanied by very little rotational and translational excitation and displays the characteristic strong, inverse temperature dependence (probability of transfer increases with decreasing temperature) expected of energy transfer mediated by a long range attractive interaction. A number of possible explanations for this apparent anomaly are considered, of which energy transfer mediated by dipole/quadrupole forces appears to be the most consistent with the data.

Reactions of N(22D) with Alkane Hydrocarbons to Produce NH(X 3Σ−)

Abstract Ground state NH radicals could be detected as products of the reactions of N(22D) with simple alkane hydrocarbons. N(2D) was produced by two-photon dissociation of NO, while NH (X 3Σ−) was detected by laser-induced fluorescence. The nascent vibrational population ratio, NH(v″ = 1)/NH(v″ = 0), was determined to be 0.6, 0.5, 0.5, and 0.5 for CH4, C2H6, C3H8, and C(CH3)4, respectively. These ratios are smaller than that for H2, 0.8, but much larger than the prior ones. The nascent rotational state distributions of NH(X 3Σ−, v″ = 0) were determined for CH4, C2H6, and C3H8. The distributions were much hotter than the prior ones and showed little dependence on the complexity of the hydrocarbons. This suggests that the intermediate complexes which lead to the production of NH are short-lived and decompose before intramolecular vibrational redistribution (ivr). The yields for the production of NH(v″ = 0) for heavy alkanes were smaller than that for CH4. There must be exit channels other than NH production, such as C–C bond cleavage processes.

Nascent rotational and vibrational state distributions of NH(X 3Σ−) and ND(X 3Σ−) produced in the reactions of N(2 2D) with H2 and D2

Two-photon dissociation of NO was employed to produce metastable atomic nitrogen N(2D) and to study its reactions. A mixture of NO and H2(D2) was irradiated with an intense laser pulse at 275.3 nm which dissociates NO to produce N(2D). Electronically ground state NH(ND) radicals could be detected as products of the N(2D)+H2(D2) reaction. The nascent rotational and vibrational state distributions of NH(ND) were determined by analyzing the laser–induced fluorescence spectra. The nascent vibrational population ratios, NH(v′′=1)/NH(v′′=0) and ND(v′′=1)/ND(v′′=0), were determined to be 0.8±0.1 and 1.0±0.1, respectively. These ratios are larger than the prior ones, but smaller than the recent results of quasiclassical trajectory calculations based on an ab initio potential energy surface. The rotational distributions of NH(ND) were very broad, both for the v′′=0 and v′′=1 levels. These results suggest that there are no specific attacking sites in these reactions.

Production of NH(X 3Σ−) radicals in the reaction of N(2 2D) with CH 4: nascent rotational and vibrational distributions of NH

Two-photon dissociation of NO was employed to produce N( 2D) and to study its reaction with CH 4. NH(X 3 Σ −) could be detected by laser-induced fluorescence. The nascent rotational and vibrational state distributions of NH were determined. The vibrational population ratio NH( ν″ = 1) /NH( ν″ = 0) was 0.6, while the rotational distributions were unimodal and hotter than the prior ones. These distributions are similar to those of OH(X 2Π) produced in the reaction of O( 1D 2) with CH 4, suggesting that N( 2D) inserts into the CH bonds.

Dynamics for the Cl+C2H6→HCl+C2H5 reaction examined through state-specific angular distributions

Photolysis of Cl2 initiates the title reaction at a sharply defined collision energy of 0.24±0.03 eV. Nascent product rotational state distributions for HCl (v=0) are determined using resonance enhanced multiphoton ionization (REMPI), center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the C2H5 product is deduced from the dependence of the core-extracted signal on the photolysis polarization. The HCl product has little rotational excitation, but the scattering distribution is nearly isotropic. Although seemingly contradictory, both of these features can be accounted for by using the simple line-of-centers model presented to explain earlier results for the Cl+CH4 reaction. In contrast to the Cl+CH4 reaction, the data suggest that the Cl+C2H6 reaction proceeds through a loosely constrained transition-state geometry. The reactions of atomic chlorine with ethane, C2H6, and perdeuteroethane, C2D6, yield virtually identical results. These findings, along with the low energy deposited by the reaction into the ethyl product (200±120 cm−1), demonstrate that the alkyl fragment acts largely as a spectator in this hydrogen abstraction reaction.

Internal energy distributions of the SO(X) 3Σ−) product from the [formula omitted] reaction

The SO(X 3 Σ −) product vibrational and rotational state distributions following the title reaction have been studied by laser induced fluorescence on the (B 3 Σ −-X 3 Σ −) transition. The O( 3P) atom was generated by 351 nm photolysis of NO 2. The SO(X 3 Σ −) product is highly vibrationally excited and the vibrational state distribution is inverted at v″=5 with detectable population up to v″=9. The observed vibrational state distribution has been modelled effectively by a Franck-Condon/Golden Rule treatment. The nascent rotational state distribution of v″=4 and 6 can be characterized by rotational temperatures, T R=2288±357 K and T R=2227±297 K, respectively. The rotational state populations are nearly equilibrated with the ambient gas within 10 hard sphere collisions. A surprisal analysis reveals that the observed rotational state distribution is colder than would be predicted on the basis of a purely statistical energy disposal model. The observed SO(X 3 Σ −) internal energy accounts for ≈ 34% of the total energy available to products. The non-statistical energy disposal results are consistent with a direct S atom abstraction mechanism.

Ultraviolet elimination of H2 from chloroethylenes

The elimination of H2 in the photodissociation of mono- and di-chloroethylenes was studied with a pump-and-probe technique. A 193 nm excimer laser was used to photodissociate the parent molecules, and a tunable dye laser was used to probe the H2 fragment by 2+1 resonance-enhanced multiphoton ionization (REMPI). The nascent rotational state distributions of H2(X 1Σ+g,v″=0–4) were extracted from the REMPI spectra, and were found to have Boltzmann-type distributions. The maximum and average translational energies for some of the rovibrational levels of H2 were measured using magic angle Doppler spectroscopy. The translational energy of the fragments plus the internal energy of H2 was found to exceed the available energy for a three-center elimination mechanism. It is concluded that a migration mechanism plays a significant role in H2 elimination.

Long- and short-range interactions in the temperature dependent collisional excitation of the antisymmetric stretching CO2(001) level by highly vibrationally excited pyrazine

The relaxation of highly vibrationally excited pyrazine, C4H4N2, by collisions with CO2 that produce molecules in the vibrationally excited antisymmetric stretch state (0001) has been investigated using high resolution infrared transient absorption spectroscopy at a series of ambient cell temperatures. The vibrationally hot (Evib≊5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the vibrationally excited CO2 molecules are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions, strongly suggest that the vibrational excitation of CO2 occurs via two mechanisms. The vibrational energy transfer is dominated by a long-range attractive force interaction, which is accompanied by almost no rotational and translational excitation. However, the CO2(0001) product state distribution also reveals a smaller contribution from a short-range interaction that results in vibrational excitation accompanied by substantial rotational and translational excitation. The long-range interaction dominates scattering into low angular momentum (J) states while the short-range interaction is most important for molecules scattering into high J states. The implications of these results for our understanding of the relaxation of molecules with chemically significant amounts of vibrational energy are discussed.

Nascent internal state distributions of ZnH(X 2Σ+) produced in the reactions of Zn(4 1P1) with some alkane hydrocarbons

The reactions of Zn(4 1P1) with CH4, C2H6, C3H8, and C(CH3)4 were studied by employing a laser pump-and-probe technique. The nascent rotational and vibrational state distributions of ZnH(X 2Σ+) were determined. These distributions were compared with those predicted by statistical models. The distributions for C(CH3)4 resembled to the statistical ones, while those for simple alkanes such as CH4 were a little hotter than the statistical ones. These results suggest that the reaction proceeds via a relatively long-lived insertive complex. There was no great difference in the production yields of ZnH, although that for CH4 was the largest.

Nascent rotational state distributions of ZnH (X 2Σ +) produced in the reactions of Zn (4 1P 1) with simple alkane hydrocarbons

The reactions of Zn(4 1P 1) with CH 4, C 2H 6, C 3H 8 and C(CH 3) 4 were studied by employing a laser pump-and-probe technique. The nascent rotational state distributions of ZnH (X 2Σ +, v″ = 0) were determined. The rotational distributions could be approximated by Boltzmann distributions in all cases. The corresponding Boltzmann temperature decreased with the size of the hydrocarbons; 9000 K for CH 4, 5000 K for C 2H 6, 2900 K for C 3H 8 and 1600 K for C (CH 3) 4. These distributions were compared with those predicted by a statistical model.

Distribution of rotational states in half-collisions: 193-nm photolysis of dichloroethylene isomers

The photodissociation of cis-, trans-, and 1,l -dichloroethylene (DCE) was studied by a pump-and-probe technique, using a 193-nm excimer laser to excite the parent molecule and time-of-flight resonance-enhanced multiphoton ionization to detect the products. We report here the nascent rotational state distributions of HCl(u”=O,1,2) and also the relative yields and intensity dependencies of HCl, H, CK2P3p), C1(2P1/2), HCP, and C1+. Our finding that the rotational state distributions of HCl for all three isomers are very similar leads us to conclude that the nascent distribution is determined far along the reaction coordinate and is insensitive to the isomeric form of the parent molecule, regardless of whether photoelimination results from a three- or four-center transition state. We also observed qualitatively different behavior for HCl(u”=O) and HCl(u”=1,2). As in our previous study of vinyl chloride, we found that HCl(u’30) has a Boltzmann-like rotational state distribution, with temperatures in this case near 1400 K, whereas the distribution for u” = 0 is described by a biexponential function with a low J” temperature around 300 K and a high J” temperature around 10 000 K. We propose that the dichotomy between these two distributions is due to microscopic rather than chemical branching. The relative yields of HCl and C1+ for trans-DCE are approximately double those of cis- and 1,l-DCE. This result is consistent with the interpretation that the yield-determining step is faster than cis-trans isomerization. The yields of all the other products are the same for cis and trans. For 1,l-DCE the yields of H, Cl(*P1p), and HCl+ are much greater than for cis- and trans-DCE, while for C1(2P3p) the 1 ,1-DCE yield is somewhat smaller. The relative yields of H and C1(2P3p) can be explained by the stabilities of the organic fragments, while the HCl+ yields can be explained statistically. All of the neutral fragments were found to be produced by singlephoton processes, while HCl+ and Cl+ require three and four photons, respectively. Ladder climbing mechanisms are proposed for the two ionic fragments.

Product energy partitioning in the unimolecular decomposition of vibrationally and rotationally state-selected hydrogen peroxide

Infrared-optical double resonance prepares HOOH molecules in single rotational levels of the 6νOH, 5νOH+νOOH, 5νOH+νOO, and 4νOH+νOH′ vibrational states which range from 3 to 2287 cm−1 of excess energy above the unimolecular dissociation threshold. Laser-induced fluorescence probes the nascent OH rotational state distributions from the decomposition of rovibrationally selected reactants. The nascent rotational state distributions reveal that both OH spin–orbit states can be populated by the decomposition of a single molecule and hence that electronic angular momentum is not conserved throughout the dissociation process. The product state distributions from reactants excited to the 6νOH and 4νOH+νOH′ vibrational levels are generally in good agreement with the predictions of phase-space theory provided electronic angular momentum is treated statistically. Reactants decomposing from single rotational states in the 5νOH+νOOH combination level (and to a lesser extent the 5νOH+νOO level) show product state distributions which are systematically colder than phase-space theory predictions. This observation indicates that energy redistribution in vibrationally excited HOOH is not complete on the time scale of unimolecular decomposition.