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Nitric oxide (●NO) participates in many biological activities, including enhancing DNA radiosensitivity in ionizing radiation-based radiotherapy. To help understand the radiosensitization of ●NO, we report reaction dynamics between ●NO and the radical cations of guanine (a 9HG●+ conformer) and 9-methylguanine (9MG●+). On the basis of the formation of 9HG●+ and 9MG●+ in the gas phase and the collisions of the radical cations with ●NO in a guided-ion beam mass spectrometer, the charge transfer reactions of 9HG●+ and 9MG●+ with ●NO were examined. For both reactions, the kinetic energy-dependent product ion cross sections revealed a threshold energy that is 0.24 (or 0.37) eV above the 0 K product 9HG (or 9MG) + NO+ asymptote. To interrogate this abnormal threshold behavior, the reaction potential energy surface for [9MG + NO]+ was mapped out at closed-shell singlet, open-shell singlet, and triplet states using density functional and coupled cluster theories. The results showed that the charge transfer reaction requires the interaction of a triplet-state surface originating from a reactant-like precursor complex 3[9MG●+(↑)⋅(↑)●NO] with a closed-shell singlet-state surface evolving from a charge-transferred complex 1[9MG⋅NO+]. During the reaction, an electron is transferred from π∗(NO) to perpendicular π∗(9MG), which introduces a change in orbital angular momentum. The latter offsets the change in electron spin angular momentum and facilitates intersystem crossing. The reaction threshold in excess of the 0 K thermochemistry and the low charge-transfer efficiency are rationalized by the vibrational excitation in the product ion NO+ and the kinetic shift arising from a long-lived triplet intermediate. [ABSTRACT FROM AUTHOR]

NO2+ in six different vibrational states was reacted with C2H2 over the center-of-mass energy range from 0.03 to 3.3 eV. The reaction, forming NO++C2H2O and NO+C2H2O+, shows a bimodal dependence on collision energy (Ecol). At low Ecol, the reaction is quite inefficient (<2%) despite this being a barrierless, exoergic reaction, and is strongly inhibited by Ecol. For Ecol>∼0.5 eV, a second mechanism turns on, with an efficiency reaching ∼27% for Ecol>3 eV. The two reaction channels have nearly identical dependence on Ecol and NO2+ vibrational state, and identical recoil dynamics, leading to the conclusion that they represent a single reaction path throughout most of the collision. All modes of NO2+ vibrational excitation enhance both channels at all Ecol, however, the effects of bend (010) and bend overtone (0200) excitation are particularly strong (factor of 4). In contrast, the asymmetric stretch (001), which intuition suggests should be coupled to the reaction coordinate, leads to only a factor of ∼2 enhancement, as does the symmetric stretch (100). Perhaps the most surprising effect is that of the bending angular momentum, which strongly suppress reaction, even though both the energy and angular momentum involved are tiny compared to the collision energy and angular momentum. The results are interpreted in light of ab initio and Rice-Ramsperger-Kassel-Marcus calculations. [ABSTRACT FROM AUTHOR]

The pulsed field ionization-photoelectron (PFI-PE) spectrum of bromochloromethane (CH2BrCl) in the region of 85 320–88 200 cm-1 has been measured using vacuum ultraviolet laser. The vibrational structure resolved in the PFI-PE spectrum was assigned based on ab initio quantum chemical calculations and Franck-Condon factor predictions. At energies 0–1400 cm-1 above the adiabatic ionization energy (IE) of CH2BrCl, the Br–C–Cl bending vibration progression (ν1+=0–8) of CH2BrCl+ is well resolved and constitutes the major structure in the PFI-PE spectrum, whereas the spectrum at energies 1400–2600 cm-1 above the IE(CH2BrCl) is found to exhibit complex vibrational features, suggesting perturbation by the low lying excited CH2BrCl+(Ã 2A″) state. The assignment of the PFI-PE vibrational bands gives the IE(CH2BrCl)=85 612.4±2.0 cm-1 (10.6146±0.0003 eV) and the bending frequencies ν1+(a1′)=209.7±2.0 cm-1 for CH2BrCl+(X 2A′). We have also examined the dissociative photoionization process, CH2BrCl+hν→CH2Cl++Br+e-, in the energy range of 11.36–11.57 eV using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0 K threshold or appearance energy AE(CH2Cl+)=11.509±0.002 eV. Combining the 0 K AE(CH2Cl+) and IE(CH2BrCl) values obtained in this study, together with the known IE(CH2Cl), we have determined the 0 K bond dissociation energies (D0) for CH2Cl+–Br (0.894±0.002 eV) and CH2Cl–Br (2.76±0.01 eV). We have also performed CCSD(T, full)/complete basis set (CBS) calculations with high-level corrections for the predictions of the IE(CH2BrCl), AE(CH2Cl+), IE(CH2Cl), D0(CH2Cl+–Br), and D0(CH2Cl–Br). The comparison between the theoretical predictions and experimental determinations indicates that the CCSD(T, full)/CBS calculations with high-level corrections are highly reliable with estimated error limits of <17 meV. [ABSTRACT FROM AUTHOR]

The effects of collision energy (Ecol) and six different H2CO+ vibrational states on the title reaction have been studied over the center-of-mass Ecol range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H2+ transfer, hydrogen transfer, and proton transfer channels is strongly affected by Ecol and H2CO+ vibrational excitation, providing insight into the factors that control competition and charge state “unmixing” during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion. [ABSTRACT FROM AUTHOR]

We report the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the reaction of H2CO+ with C2D4 over the center-of-mass Ecol range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. Seven product channels are observed. Charge transfer (CT) has the largest cross section over the entire energy range, substantially exceeding the hard sphere cross section at high energies. Competing with CT are six channels involving transfer of one or more hydrogen atoms or protons and one involving formation of propanal, followed by hydrogen elimination. Despite the existence of multiple deep wells on the potential surface, all reactions go by direct mechanisms, except at the lowest collision energies, where short-lived complexes appear to be important. Statistical complex decay appears adequate to account for the product branching at low collision energies, however, even at the lowest energies, the vibrational effects are counter to statistical expectations. The pattern of Ecol and vibrational mode effects provide insight into factors that control reaction and interchannel competition. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the reaction of H2CO+ with C2D4 over the center-of-mass Ecol range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. Seven product channels are observed. Charge transfer (CT) has the largest cross section over the entire energy range, substantially exceeding the hard sphere cross section at high energies. Competing with CT are six channels involving transfer of one or more hydrogen atoms or protons and one involving formation of propanal, followed by hydrogen elimination. Despite the existence of multiple deep wells on the potential surface, all reactions go by direct mechanisms, except at the lowest collision energies, where short-lived complexes appear to be important. Statistical complex decay appears adequate to account for the product branching at low collision energies, however, even at the lowest energies, the vibrational effects are counter to statistical expectations. The pattern of Ecol and vibrational mode effects provide insight into factors that control reaction and interchannel competition. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effects on the title reaction of collision energy (Ecol) and five H2CO+ vibrational modes have been studied over a center-of-mass Ecol range from 0.1 to 2.3 eV. Electronic structure and Rice–Ramsperger–Kassel–Marcus calculations were used to examine properties of various complexes and transition states that might be important. Only the hydrogen abstraction (HA) product channel is observed, and despite being exoergic, HA has an appearance energy of ∼0.4 eV, consistent with a transition state found in the electronic structure calculations. A precursor complex-mediated mechanism might possibly be involved at very low Ecol, but the dominant mechanism is direct over the entire Ecol range. The magnitude of the HA cross section is strongly, and mode specifically affected by H2CO+ vibrational excitation, however, vibrational energy has no effect on the appearance energy. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quasiclassical, direct dynamics trajectories, calculated at the B3LYP/6-31G** level of the theory, have been used to study the energy transfer dynamics and collision-induced dissociation (CID) of formaldehyde cation in collisions with Ne. Effects of varying collision energy were probed for ground state H[SUB2]CO[SUP+], and H[SUB2]CO[SUP+] with excitation in three different vibrational modes: ν[SUP+,SUB5] (in-plane CH[SUB2] rock), ν[SUP+,SUB4] (out-of-plane bend), and ν[SUP+,SUB5] (asym. CH stretch). The trajectories are in excellent agreement with an earlier detailed experimental study of state-selected H[SUB2]CO[SUP+] CID. Energy disposal branching between product recoil, vibrational energy, and rotational excitation is found to vary with collision energy, vibrational state, impact parameter, and orientation. An analysis of the trajectories illustrates how mode-specific vibrational excitation effects the efficiency of translational-to-vibrational (T→V) energy transfer to promote CID. [ABSTRACT FROM AUTHOR]

The effects on the title reaction of collision energy (E[SUBcol]), five H[SUB2]CO[SUP+] vibrational modes, and deformation vibrations of methane have been studied, including the measurement of product integral and differential cross sections over a center-of-mass E[SUBcol] range from 0.09-3.3 eV. Electronic structure and RRKM calculations are reported, providing an additional mechanistic insight. The total reaction efficiency is well below unity, despite there being two exoergic reaction pathways with no activation barriers. The energetically more favorable channel corresponds to H elimination (HE) from an intermediate complex, however, this channel accounts for only ∼15% of the total reaction cross section at low E[SUBcol] and is negligible at high energies. The dominant channel, hydrogen abstraction (HA) by H[SUB2]CO[SUP+] from methane, is dominated by a complex-mediated mechanism at low E[SUBcol], switching over to a direct hydrogen-stripping mechanism at high E[SUBcol]. Both HA and HE are inhibited in a strongly mode-specific fashion by H[SUB2]CO[SUP+] vibrational excitations, and greatly enhanced by excitation of methane deformation vibrations. The strong mode specificity indicates that the reaction-limiting step occurs early in the collisions. [ABSTRACT FROM AUTHOR]

Vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for CS[SUB2] have been obtained in the energy range of 13.2-17.6 eV, revealing complex vibronic structures for the CS[SUB2,SUP+](&Btilde;[SUP2]∑[SUBu,SUP+] and &Ctilde;[SUP2]∑[SUBg,SUP+]) states. The PFI-PE spectra for CS[SUB2,SUP+](&Btilde;[SUP2]∑[SUBu,SUP+] and &Ctilde;[SUP2]∑[SUBg,SUP+]) are dominated by v[SUB1,SUP+] (symmetric stretching) mode. However, PFI-PE bands due to the v[SUB2,SUP+] (bending) and v[SUB3,SUP+] (antisymmetric stretching) modes with both even and odd quanta are clearly resolved. The simulation of rotational contours resolved in the origin PFI-PE bands yields accurate ionization energies of 14.4742±0.0005 eV (116742±4 cm[SUP-1]) and 16.1883±0.0005 eV (130567±4 cm[SUP-1]) for the formation of CS[SUB2,SUP+](&Btilde;[SUP2]∑[SUBu,SUP+] and &Ctilde;[SUP2]∑[SUBg,SUP+]) states from CS[SUB2](&Xtilde;[SUP1]∑[SUBg,SUP+]), respectively. The PFI-PE bands for [SUP2]∑[SUBu,SUP+](3[SUB0,SUP3]) and [SUP2]∑[SUBg,SUP+](3[SUB0,SUP3]) at 14.805 and 14.965 eV, which are in near energy resonance with the 0 K dissociation thresholds for the formation of S[SUP+]([SUP4]S)+CS(&Xtilde;[SUP1]∑[SUP+]; v"=0 and 1) from CS[SUB2], respectively, are found to be enhanced. These enhancements are rationalized by the prompt dissociation of excited CS[SUB2] in high-n (n&ges;100) Rydberg states prior to PFI. Three-dimensional potential energy functions (PEFs) for CS[SUB2,SUP+](2[SUP2]II[SUBu], &Btilde;[SUP2]∑[SUBu,SUP+], and &Ctilde;[SUP2]∑[SUBg,SUP+]) states have been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. On the basis of these PEFs, the harmonic frequencies for CS[SUB2,SUP+](2[SUP2]II[SUBu], (&Btilde;[SUP2]∑[SUBu,SUP+] and &Ctilde;[SUP2]∑[SUBg,SUP+]) and vibronic energy levels for CS[SUB2,SUP+](&Btilde... [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet pulsed field ionization (PFI)-photoelectron (PFI-PE) spectrum of CO[sub 2] has been measured in the energy region of 19.0-20.0 eV. The PFI-PE vibrational bands resolved for CO[sup +, sub 2] are overwhelmingly dominated by the origin band along with weak vibrational bands corresponding to excitations of the v[sup +, sub 1] (symmetric stretching), v[sup +, sub 2] (bending), and v[sup +, sub 3] (antisymmetric stretching) modes. The simulation of the rotational contour resolved in the origin PFI-PE band yields a value of 19.3911±0.0005 eV for the ionization energy of CO[sub 2] to form CO[sup +, sub 2](C [sup 2]&Zigma;[sup +, sub g])./ A PFI-PE peak is found to coincide with each of the 0 K dissociation thresholds for the formation of O[sup +]([sup 4]S) + CO(X [sup 1]&Zigma;[sup +]) and CO[sup +] (X ²&Zigma;[sup +] ) +O(³P). This observation is tentatively interpreted to result from the lifetime switching effect, arising from the prompt dissociation of excited CO[sub 2] in high-n (n ≥ 100) Rydberg states prior to PFI. We have also examined the decay pathways for state-selected CO[sup +, sub 2] in the internal energy range of 5.2-6.2 eV using the PFI-PE-photoion coincidence scheme. The coincidence TOF data show unambiguously the formation of O[sup +]([sup 4]S) + CO(X 1&Zigma;[sup +]; v″ = 0,1) and CO[sup +](X ²&Zigma;[sup +] ; v[sup +] = 0,1) + O(³P). Analysis of the kinetic energy releases of fragment ions suggests that the dissociation of excited CO[sup +, sub 2] involved is nonstatistical and proceeds with an impulsive mechanism. Potential energy functions (PEFs) for the CO[sup +, sub 2] (C ²&Zigma;[sup +, sub g]) state and the lowest quartet states of CO[sup +, sub 2], together with their spin-orbit interactions, have been calculated using the complete active space self-consistent field and internal contracted multireference configuration... [ABSTRACT FROM AUTHOR]

The effects of collision energy (E[SUBcol]) and five different H[SUB2]CO[SUP+] vibrational modes on the title reaction have been studied, including measurements of product ion recoil velocity distributions. A series of ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine properties of various complexes and transition states that might be important. Four product channels are observed. Proton transfer (PT) dominates at low E[SUBcol], and is suppressed by E[SUBcol] but mildly enhanced by H[SUB2]CO[SUP+] vibrational excitation. PT occurs by a direct mechanism at high energies, but appears to be mediated by reactantlike complexes at low energies. The other major low energy channel corresponds to H[SUP+,SUB2] transfer, and the majority of these product ions go on to eliminate CO, producing H[SUB2]S[SUP+]. Both H[SUP+,SUB2] transfer and H[SUB2]S[SUP+] channels are strongly inhibited by E[SUBcol] and vibrational excitation, which is interpreted in terms of competition with other channels. Charge transfer occurs in short time scale collisions at all energies, and is strongly enhanced by E[SUBcol] and by vibrational excitation. The vibrational effects for all channels are mode specific. [ABSTRACT FROM AUTHOR]

We report a study of collision-induced dissociation (CID) of H[sub 2]CO[sup +], including measurement of the effects of collision energy (E[sub co1)] and five different H[sub 2]CO[sup +] vibrational modes on the CID integral and differential cross sections. CID was studied for collision with both Xe and Ne, and the Ne results provide a very detailed probe of energy transfer collisions leading to CID. The CID appearance threshold is found to depend only on total energy, but for all energies above threshold, vibrational energy is far more effective at driving CID than E[sub co1], with some mode-specificity. Results are fit with an impact parameter-based mechanism, and considerable insight is obtained into the origins of the E[sub co1] and vibrational effects. A series of ab initio and RRKM calculations were also performed to help interpret the results. [ABSTRACT FROM AUTHOR]

Using the high-resolution pulsed field ionization-photoelectron (PFI-PE) and PFI-PE-photoion coincidence (PFI-PEPICO) techniques, we have examined the formation of methyl cation (CH[sub 3][sup +]) from the dissociation of energy-selected CH[sub 3]X[sup +] (X=Br and I) near their dissociation thresholds. The breakdown diagrams for CH[sub 3]X thus obtained yield values of 12.834±0.002 eV and 12.269±0.003 eV for the 0 K dissociative threshold or appearance energy (AE) for CH[sub 3][sup +] from CH[sub 3]Br and CH[sub 3]I, respectively. Similar to the observation in PFI-PE studies of CH[sub 4], C[sub 2]H[sub 2], and NH[sub 3], the PFI-PE spectrum for CH[sub 3]Br exhibits a step at the 0 K AE for CH[sub 3][sup +], indicating that the dissociation of excited CH[sub 3]Br in high-n (>=100) Rydberg states at energies slightly above the dissociation threshold occurs in a time scale of ≤10[sup -7] s. The observed step is a confirmation of the 0 K AE(CH[sub 3][sup +]) from CH[sub 3]Br determined in the PFI-PEPICO study. The adiabatic ionization energies (IEs) for the CH[sub 3]Br[sup +](X˜ [sup 2]E[sub 3/2,1/2]) spin–orbit states were determined by PFI-PE measurements to be 10.5427±0.0010 and 10.8615±0.0010 eV, respectively, yielding the spin–orbit coupling constant to be 2571±4 cm-1. The AE(CH[sub 3][sup +]) values from CH[sub 3]Br and CH[sub 3]I and the IE[CH[sub 3]Br[sup +](X˜ [sup 2]E[sub 3/2])] value obtained here, when combined with the known IE of CH[sub 3] (9.8380±0.0004 eV) and IE[CH[sub 3]I[sup +](X˜ [sup 2]E[sub 3/2])] (9.5381±0.0001 eV), have allowed accurate determination of the 0 K bond dissociation energies for CH[sub 3]–Br (2.996±0.002 eV), CH[sub 3][sup +]–Br (2.291±0.002 eV), CH[sub 3]–I (2.431±0.003 eV), and CH[sub 3][sup +]–I (2.731±0.003 eV). Using the AE(CH[sub 3][sup +]) from CH[sub 3]Br and CH[sub 3]I, together with the known 0 K he... [ABSTRACT FROM AUTHOR]

Integral cross sections and product velocity distributions were measured for reaction of acetaldehyde cation with methanol over a center-of-mass collision energy range from 0.1 to 2.2 eV. Reactivity is dominated by exoergic proton transfer (PT), which is strongly suppressed by collision energy, and mildly suppressed by CH[sub 3]CHO[sup +] vibrational excitation. PT is complex-mediated at low energies, switching to a direct stripping mechanism at high energies. Of the two possible PT channels, it appears that transfer of the aldehyde proton dominates. Hydrogen abstraction (HA) is a minor channel at low collision energies, also complex-mediated. Abstraction is observed from both hydroxyl and methyl sites on methanol, and the two channels have different, and counterintuitive collision energy dependence. Despite being exoergic, with no barriers, the HA channel shows apparent threshold behavior, attributed to competition with the dominant PT channel. The competition indicates that different intermediate complexes must interconvert efficiently, at least for low collision energies. At low energies, HA is strongly enhanced by collision energy, while vibrational excitation has no effect. Finally, there is a minor product channel corresponding to methyl elimination (ME) from a complex. Despite a relatively complicated reaction coordinate, the ME channel shows substantial recoil energy release and an asymmetric velocity distribution. A series of ab initio and RRKM calculations were performed to help interpret the results. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Employing the newly developed high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, we have examined the dissociation of energy-selected NH[sub 3][sup +] to form NH[sub 2][sup +]+H near its threshold. The breakdown curves for NH[sub 2][sup +] and NH[sub 3][sup +] thus obtained yield a value of 15.765±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for NH[sub 2][sup +] from NH[sub 3]. This value, together with the known ionization energy (IE=10.1864±0.0001 eV) and 0 K bond dissociation energy (D[sub 0]=4.6017±0.0025 eV) for NH[sub 3], allows the determination of the D[sub 0](NH[sub 2][sup +]–H) and IE(NH[sub 2]), which are 5.5786±0.0010 and 11.1633±0.0025 eV, respectively. Using the known 0 K heats of formation (ΔH[sub f0][sup °]) for NH[sub 3] and H and the AE(NH[sub 2][sup +]), we obtain the ΔH[sub f0][sup °](NH[sub 2][sup +])=302.60±0.08 kcal/mol. The PFI-PE spectrum for NH[sub 3] exhibits a step-like feature at the 0 K AE(NH[sub 2][sup +]), indicating that the dissociation of excited NH[sub 3] in high-n (n>=100) Rydberg states at energies slightly above the dissociation threshold occurs on a time scale ≤10[sup -7] s. This step confirms the AE(NH[sub 2][sup +]) value derived from the PFI-PEPICO measurements. Highly accurate energetic data with well-founded error limits, such as those obtained in the present and other studies using the PFI techniques, are expected to play an important role for the development of the next generation of ab initio quantum computation procedures. This experiment has stimulated a state-of-the-art ab initio quantum chemical calculation [Dixon et al., J. Chem. Phys. 115, 2576 (2001)]. The comparison between theoretical predictions and the best experimental results for the NH[sub 2]/NH[sub 2][sup +] and NH[sub 3]/NH[sub 3][sup +] systems indicates that the accuracy of the computational scheme used ... [ABSTRACT FROM AUTHOR]

Scattering of mode-selectively excited acetaldehyde cations from D[sub 2]O was studied in a guided ion beam instrument. The effects of reactant vibrational state and collision energy on reactivity, product branching, and product ion recoil velocity distributions were measured. Ab initio calculations were performed to help understand the reaction coordinate. The dominant reaction is H/D exchange, which occurs in about 40% of low energy collisions, dropping to just a few percent at high energies. H/D exchange is also inhibited by CH[sub 3]CHO[sup +] vibration, but with a smaller effect than the equivalent amount of collision energy. H/D exchange is mediated by a long-lived complex, and several candidates are identified. The other low energy channel corresponds to methyl elimination from the collision complex. This channel is the most energetically favorable, but is only a few percent efficient, even at low energies, and is negligible at high energies. Methyl elimination is strongly suppressed by both collision energy and vibration, and the vibrational effects are nonmode specific. The most interesting channel is proton transfer (PT), which occurs by a direct mechanism at all collision energies. At low energies, PT occurs only in small impact parameter collisions, while at high energies, PT occurs primarily for large impact parameters, and is suppressed for small impact parameters. PT also shows strongly mode-specific dependence on CH[sub 3]CHO[sup +] vibrational state. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The resonance-enhanced multiphoton ionization (REMPI) spectrum of formaldehyde, two photon resonant in the region of the [sup 1]A[sub 2](3p[sub x]), [sup 1]A[sub 1](3p[sub y]), and [sup 1]B[sub 2](3p[sub z]) states, is reported. The [sup 1]A[sub 2](3p[sub x]) state spectrum is dominated by the ν[sub 3][sup ′] (CH[sub 2] scissors), ν[sub 4][sup ′] (CH[sub 2] out-of-plane bending), ν[sub 5][sup ′] (CH[sub 2] asymmetric stretching), and ν[sub 6][sup ′] (CH[sub 2] rock) modes, with weaker bands observed for excitation of the ν[sub 2][sup ′] (CO stretching) mode. Vibrational analysis of the spectrum provides many new frequencies for the [sup 1]A[sub 2](3p[sub x]) state, not resolved or accessible in single photon spectroscopic measurements. Photoelectron spectroscopy is used to probe the nature of the vibronic levels associated with the [sup 1]A[sub 2](3p[sub x]) intermediate state, to measure vibrational frequencies of the resulting cations, and to identify useful routes for preparing vibrational state-selected H[sub 2]CO[sup +]. It is found that H[sub 2]CO[[sup 1]A[sub 2](3p[sub x])] is a well-behaved Rydberg state, generating cations in the same vibrational level that was populated in the intermediate. Cations with mode-selective excitation of up to 0.62 eV can be produced. Ab initio calculations are used to help assign the cation vibrations. In contrast to the well-behaved [sup 1]A[sub 2](3p[sub x]) state, the [sup 1]A[sub 1](3p[sub y]) and [sup 1]B[sub 2](3p[sub z]) states are strongly mixed with each other and with valence states. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reaction of acetaldehyde cations with acetylene has been studied as a function of collision energy and acetaldehyde vibrational state. CH[sub 3]CHO[sup +] is state selected by resonance-enhanced multiphoton ionization, and scattered from C[sub 2]H[sub 2] in a guided ion beam instrument. We have also calculated the structures and energetics of 11 different hydrogen-bonded, covalently bound, and coordination complexes, some of which are clearly intermediates in the reaction. From the product distribution, it is clear that some of most stable complexes do not participate in the reaction. The most exoergic product observed corresponds to CH[sub 3] elimination from a covalently bound complex, but this channel accounts for no more than a few percent of the total reaction signal. The dominant channel is hydrogen transfer (CH[sub 3]CO[sup +]+C[sub 2]H[sub 3]), which proceeds with >73% efficiency at low energies, dropping to a near constant 30% at energies above 1 eV. Product recoil velocity distributions indicate that this channel is complex mediated at low energies, switching to direct scattering at high energies. The hydrogen transfer reaction is weakly affected by reactant vibration, including ν[sub 3], the aldehyde CH stretch. Methyl elimination is strongly, but nonmode specifically inhibited by vibration. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Resonance-enhanced multiphoton ionization (REMPI) and photoelectron spectroscopy (PES), have been used to study the [sup 1]A[sub g](3dπ) Rydberg state of 1,3-trans-butadiene in the two photon energy range from 61 000 to 66 400 cm-1. The [sup 1]A[sub g](3dπ) spectrum is dominated by the ν[sub 4][sup ′], ν[sub 6][sup ′], and ν[sub 9][sup ′] vibrational modes, with some excitation of the ν[sub 8][sup ′] mode, as well. Photoelectron spectroscopy shows that the dominant ionization pathways are diagonal, i.e., they produce cations in the same vibrational level that was populated in the Rydberg state. Weaker off-diagonal ionization is also observed, with excitation of the ν[sub 4][sup +], ν[sub 6][sup +], and ν[sub 9][sup +] modes. The relative intensities of diagonal and off-diagonal PES bands are observed to be strongly dependent on the angle between the laser polarization and the detection axis. It is possible to use REMPI to generate state-selected cations, however, the nascent ions are quite efficiently photodissociated by the REMPI laser. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet (VUV) pulsed field ionization-photoelectron (PFI-PE) spectra for CO[sub 2] have been measured in the energy range of 17.2-19.0 eV, showing complex vibronic structures for the CO[sub 2][sup +](A [sup 2]Π[sub u] and B [sup 2]Σ[sub u][sup +]) states. The PFI-PE spectra for CO[sub 2][sup +](A [sup 2]Π[sub u] and B [sup 2]Σ[sub u][sup +]) are dominated by the v[sub 1][sup +] (symmetric stretching) vibrational progressions, and weak bands due to excitation of both even and odd quanta of the v[sub 2][sup +] (bending) and v[sub 3][sup +] (antisymmetric stretching) modes are observed in the VUV-PFI-PE spectra. The simulation of rotational contours resolved in the PFI-PE vibronic bands associated with excitation to CO[sub 2][sup +](A [sup 2]Π[sub 3/2,1/2u];v[sub 1][sup +]=0-5,v[sub 2][sup +]=0,v[sub 3][sup +]=0) and CO[sub 2][sup +](B [sup 2]Σ[sub u][sup +];0,0,0) has yielded accurate ionization energies for the formation of these states from CO[sub 2](X [sup 1]Σ[sub g][sup +]). Three-dimensional potential energy functions (PEFs) for CO[sub 2][sup +](B [sup 2]Σ[sub u][sup +]) have also been generated theoretically using the complete active space self-consistent field and internally contracted multireference configuration interaction methods. Based on these PEFs, vibrational energy levels for CO[sub 2][sup +](B [sup 2]Σ[sub u][sup +]), together with the Franck-Condon factors for their formation from CO[sub 2](X [sup 1]Σ[sub g][sup +]), have been calculated. With the guide of these theoretical predictions, the vibrational bands resolved in the PFI-PE spectrum for CO[sub 2][sup +](B [sup 2]Σ[sub u][sup +]) have been satisfactorily assigned. This assignment reveals the nature of many vibrational PFI-PE bands as originated from anharmonic resonance interactions and members of Fermi polyads. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vacuum ultraviolet pulsed field ionization-photoelectron (PFI-PE) spectra for CO[sub 2] have been measured in the energy range of 13.6-14.7 eV, revealing complex vibronic structures for the ground CO[sub 2][sup +](X [sup 2]Π[sub g]) state. Many vibronic bands for CO[sub 2][sup +](X [sup 2]Π[sub g]), which were not resolved in previous photoelectron studies, are identified in the present measurement based on comparison with available optical data and theoretical predictions. As observed in the HeI photoelectron spectrum of CO[sub 2], the PFI-PE spectrum is dominated by the symmetry allowed ν[sub 1][sup +] (symmetric stretch) vibrational progression for CO[sub 2][sup +](X [sup 2]Π[sub g]). However, PFI-PE vibronic bands due to excitation of the symmetry disallowed ν[sub 2][sup +] (bending) and ν[sub 3][sup +] (asymmetric stretch) modes with both odd quanta, together with the symmetry allowed even quanta excitations, are clearly discernible. The simulation of rotational contours resolved in PFI-PE vibronic bands associated with excitation to the (ν[sub 1][sup +]=0-1, ν[sub 2][sup +]=0-2, ν[sub 3][sup +]=0) vibrational levels has yielded accurate ionization energies for the formation of these vibronic states from CO[sub 2](X [sup 1]Σ[sub g][sup +]). © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

NO(2)(+) in six different vibrational states was reacted with C(2)H(2) over the center-of-mass energy range from 0.03 to 3.3 eV. The reaction, forming NO(+)+C(2)H(2)O and NO+C(2)H(2)O(+), shows a bimodal dependence on collision energy (E(col)). At low E(col), the reaction is quite inefficient (<2%) despite this being a barrierless, exoergic reaction, and is strongly inhibited by E(col). For E(col)> approximately 0.5 eV, a second mechanism turns on, with an efficiency reaching approximately 27% for E(col)>3 eV. The two reaction channels have nearly identical dependence on E(col) and NO(2)(+) vibrational state, and identical recoil dynamics, leading to the conclusion that they represent a single reaction path throughout most of the collision. All modes of NO(2)(+) vibrational excitation enhance both channels at all E(col), however, the effects of bend (010) and bend overtone (02(0)0) excitation are particularly strong (factor of 4). In contrast, the asymmetric stretch (001), which intuition suggests should be coupled to the reaction coordinate, leads to only a factor of approximately 2 enhancement, as does the symmetric stretch (100). Perhaps the most surprising effect is that of the bending angular momentum, which strongly suppress reaction, even though both the energy and angular momentum involved are tiny compared to the collision energy and angular momentum. The results are interpreted in light of ab initio and Rice-Ramsperger-Kassel-Marcus calculations.

The pulsed field ionization-photoelectron (PFI-PE) spectrum of bromochloromethane (CH2BrCl) in the region of 85,320-88,200 cm-1 has been measured using vacuum ultraviolet laser. The vibrational structure resolved in the PFI-PE spectrum was assigned based on ab initio quantum chemical calculations and Franck-Condon factor predictions. At energies 0-1400 cm-1 above the adiabatic ionization energy (IE) of CH2BrCl, the Br-C-Cl bending vibration progression (nu1+=0-8) of CH2BrCl+ is well resolved and constitutes the major structure in the PFI-PE spectrum, whereas the spectrum at energies 1400-2600 cm-1 above the IE(CH2BrCl) is found to exhibit complex vibrational features, suggesting perturbation by the low lying excited CH2BrCl+(A 2A") state. The assignment of the PFI-PE vibrational bands gives the IE(CH2BrCl)=85,612.4+/-2.0 cm-1 (10.6146+/-0.0003 eV) and the bending frequencies nu1+(a1')=209.7+/-2.0 cm-1 for CH2BrCl+(X2A'). We have also examined the dissociative photoionization process, CH2BrCl+hnu-->CH2Cl++Br+e-, in the energy range of 11.36-11.57 eV using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0 K threshold or appearance energy AE(CH2Cl+)=11.509+/-0.002 eV. Combining the 0 K AE(CH2Cl+) and IE(CH2BrCl) values obtained in this study, together with the known IE(CH2Cl), we have determined the 0 K bond dissociation energies (D0) for CH2Cl+-Br (0.894+/-0.002 eV) and CH2Cl-Br (2.76+/-0.01 eV). We have also performed CCSD(T, full)/complete basis set (CBS) calculations with high-level corrections for the predictions of the IE(CH2BrCl), AE(CH2Cl+), IE(CH2Cl), D0(CH2Cl+-Br), and D0(CH2Cl-Br). The comparison between the theoretical predictions and experimental determinations indicates that the CCSD(T, full)/CBS calculations with high-level corrections are highly reliable with estimated error limits of <17 meV.

A combined experimental and trajectory study of vibrationally state-selected NO2+ collisions with Ne, Ar, Kr, and Xe is presented. Ne, Ar, and Kr are similar in that only dissociation to the excited singlet oxygen channel is observed; however, the appearance energies vary by approximately 4 eV between the three rare gases, and the variation is nonmonotonic in rare gas mass. Xe behaves quite differently, allowing efficient access to the ground triplet state dissociation channel. For all four rare gases there are strong effects of NO2+ vibrational excitation that extend over the entire collision energy range, implying that vibration influences the efficiency of collision to internal energy conversion. Bending excitation is more efficient than stretching; however, bending angular momentum partially counters the enhancement. Direct dynamics trajectories for NO2+ + Kr reproduce both the collision energy and vibrational state effects observed experimentally and reveal that intracomplex charge transfer is critical for the efficient energy transfer needed to drive dissociation. The strong vibrational effects can be rationalized in terms of bending, and to a lesser extent, stretching distortion enhancing transition to the Kr+ -NO2 charge state.

The effects of collision energy (E(col)) and six different H(2)CO(+) vibrational states on the title reaction have been studied over the center-of-mass E(col) range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H(2) (+) transfer, hydrogen transfer, and proton transfer channels is strongly affected by E(col) and H(2)CO(+) vibrational excitation, providing insight into the factors that control competition and charge state "unmixing" during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion.

We report the effects of collision energy (Ecol) and five different H2CO+ vibrational modes on the reaction of H2CO+ with C2D4 over the center-of-mass E(col) range from 0.1 to 2.1 eV. Properties of various complexes and transition states were also examined computationally. Seven product channels are observed. Charge transfer (CT) has the largest cross section over the entire energy range, substantially exceeding the hard sphere cross section at high energies. Competing with CT are six channels involving transfer of one or more hydrogen atoms or protons and one involving formation of propanal, followed by hydrogen elimination. Despite the existence of multiple deep wells on the potential surface, all reactions go by direct mechanisms, except at the lowest collision energies, where short-lived complexes appear to be important. Statistical complex decay appears adequate to account for the product branching at low collision energies, however, even at the lowest energies, the vibrational effects are counter to statistical expectations. The pattern of Ecol and vibrational mode effects provide insight into factors that control reaction and interchannel competition., ((c) 2004 American Institute of Physics)

The effects on the title reaction of collision energy (E(col)) and five H(2)CO(+) vibrational modes have been studied over a center-of-mass E(col) range from 0.1 to 2.3 eV. Electronic structure and Rice-Ramsperger-Kassel-Marcus calculations were used to examine properties of various complexes and transition states that might be important. Only the hydrogen abstraction (HA) product channel is observed, and despite being exoergic, HA has an appearance energy of approximately 0.4 eV, consistent with a transition state found in the electronic structure calculations. A precursor complex-mediated mechanism might possibly be involved at very low E(col), but the dominant mechanism is direct over the entire E(col) range. The magnitude of the HA cross section is strongly, and mode specifically affected by H(2)CO(+) vibrational excitation, however, vibrational energy has no effect on the appearance energy., ((c) 2004 American Institute of Physics.)