In a recent letter (J. Phys. Chem. A, 2001, 105,1), we argued that, although all major thermochemical tables recommend a value of ∆H°f 0 (OH) based on a spectroscopic approach, the correct value is 0.5 kcal/mol lower as determined from an ion cycle. In this paper, we expand upon and augment both the experimental and theoretical arguments presented in the letter. In particular, three separate experiments (mass-selected photoionization measurements, pulsed-field-ionization photoelectron spectroscopy measurements, and photoelectron-photoion coincidence measurements) utilizing the positive ion cycle to derive the O-H bond energy are shown to converge to a consensus value of the appearance energy AE 0 (OH(18.116 2 ( 0.003 0 eV). With the most accurate currently available zero kinetic energy photoionization value for the ionization energy IE(OH) ) 104989 ( 2 cm -1 , corroborated by a number of photoelectron measurements, this leads to D 0 (H-OH) ) 41128 ( 24 cm -1 ) 117.59 ( 0.07 kcal/mol. This corresponds to ∆H f0 (OH) ) 8.85 ( 0.07 kcal/mol and implies D 0 (OH) ) 35593 ( 24 cm -1 ) 101.76 ( 0.07 kcal/mol. These results are completely supported by the most sophisticated theoretical calculations ever performed on the H x O system, CCSD(T)/aug-cc-pVnZ, n ) Q, 5, 6, and 7, extrapolated to the CBS limit and including corrections for core-valence effects, scalar relativistic effects, incomplete correlation recovery, and diagonal Born-Oppenheimer corrections. These calculations have an estimated theoretical error of e0.2 kcal/mol based on basis set convergence properties. They reproduce the experimental results for dissociation energies, atomization energies, and ionization energies for the H x O system to within 0.0-0.2 kcal/mol. In contrast, the previously accepted values of the two successive bond dissociation energies of water differ from the current values by 0.5 kcal/mol. These values were derived from the spectroscopic determinations of D 0 (OH) using a very short Birge-Sponer extrapolation on OH/OD A 1 Σ + . However, on the basis of a calculation of the A state potential energy curve (with a multireference single and double excitation wave function and an augcc-pV5Z basis set) and an exhaustive reanalyzis of the original measured data on both the A and B states of OH, the Birge-Sponer extrapolation can be demonstrated to significantly underestimate the bond dissociation energy, although only the last vibrational level was not observed experimentally. The recommended values of this paper affect a large number of other thermochemical quantities which directly or indirectly rely on or refer to D 0 (H-OH), D 0 (OH), or ∆H°f(OH). This is illustrated by an analysis of several reaction enthalpies, deprotonation enthalpies, and proton affinities.

At the Advanced Light Source an undulator beamline, with an energy range from 6 to 30 eV, has been constructed for chemical dynamics experiments. The higher harmonics of the undulator are suppressed by a novel, windowless gas filter. In one branchline high-flux, 2% bandwidth radiation is directed toward an end station for photodissociation and crossed molecular-beam experiments. A photon flux of 10 16 photon/s has been measured at this end station. In a second branchline a 6.65 m off-plane Eagle monochromator delivers narrow bandwidth radiation to an end station for photoionoization studies. At this second end station a peak flux of 3 ϫ 10 11 was observed for 25 000 resolving power. This monochromator has achieved a resolving power of 70 000 using a 4800 grooves/mm grating, one of the highest resolving powers obtained by a vacuum ultraviolet monochromator.

Photodissociation of carbon dioxide (CO2) has long been assumed to proceed exclusively to carbon monoxide (CO) and oxygen atom (O) primary products. However, recent theoretical calculations suggested that an exit channel to produce C + O2 should also be energetically accessible. Here we report the direct experimental evidence for the C + O2 channel in CO2 photodissociation near the energetic threshold of the C(3P) + O2(X3Σg–) channel with a yield of 5 ± 2% using vacuum ultraviolet laser pump-probe spectroscopy and velocity-map imaging detection of the C(3PJ) product between 101.5 and 107.2 nanometers. Our results may have implications for nonbiological oxygen production in CO2-heavy atmospheres.

The recent developments of vacuum ultraviolet (VUV) laser and third generation synchrotron radiation sources, together with the introduction of pulsed field ionization (PFI) schemes for photoion-photoelectron detection, have had a profound impact on the field of VUV spectroscopy and chemistry. Owing to the mediation of near-resonant autoionizing states, rovibronic states of ions with negligible Franck-Condon factors for direct photoionization can be examined by VUV-PFI measurements with rotational resolutions. The VUV-PFI spectra thus obtained have provided definitive ionization energies (IEs) for many small molecules. The recent synchrotron-based PFI-photoelectron-photoion coincidence experiments have demonstrated that dissociative photoionization thresholds for a range of molecules can be determined to the same precision as in PFI-photoelectron measurements. Combining appropriate dissociation thresholds and IEs measured in PFI studies, thermochemical data for many neutrals and cations can be determined with unprecedented precision. The further development of two-color excitation-ionization schemes promises to expand the scope of spectroscopic and chemical applications using the photoionization-photoelectron method.

The pulsed-field ionization zero-electron kinetic-energy ͑PFI-ZEKE͒ threshold photoionization spectrum of NO 2 from 9.58 to 20 eV is obtained using vacuum ultraviolet synchrotron radiation by means of the Chemical Dynamics Beamline at the Lawrence Berkeley National Laboratory Advanced Light Source. The high resolution afforded by PFI threshold discrimination yields new or refined spectroscopic constants for a number of known excited states of the cation, including the first estimate of the A rotational constant in the a 3 B 2 state, as well as new fundamental frequencies for the A 1 A 2 and B 1 B 2 states, a precise determination of the singlet-triplet splitting in the c 3 B 1 -C 1 B 1 complex and the first observations of the states, d 3 A 1 and D 1 B 2 . Most significantly, ZEKE photoelectron detection resolves vibrational structure in the linear X 1 ⌺ g ϩ ground state of NO 2 ϩ . Vibrational positions in the first electron volt of the spectrum are found to conform with the predictions of a Hamiltonian that includes Fermi resonance and other anharmonic terms derived from earlier multiresonant laser spectroscopic experiments on the lower bending excited states.

A unique triple-quadrupole double-octopole (TQDO) photoionization mass spectrometer has been developed for total cross section measurements of state-selected ion-molecule reactions. By employing this TQDO apparatus, we have recently examined the absolute total cross sections for a series of state-selected ionmolecule reactions involving Ar + ( 2 P 3/2,1/2 ), O + ( 4 S, 2 D, 2 P), and organosulfur ions (CH 3 SH + , CH 3 CH 2 SH + , and CH 3 SCH 3 + ) in their ground states. The cross section measurements, together with product ion kinetic energy analyses, have provided convincing evidence that the Ar + ( 2 P 3/2,1/2 ) + CO 2 (CO, N 2 , O 2 ) reactions proceed via a charge-transfer predissociation mechanism. The comparison of absolute cross sections for product ions formed in the dissociative charge transfer of Ar + ( 2 P 3/2,1/2 ) + CO 2 (CO, N 2 , O 2 ) and those produced in photoionization of CO 2 (CO, N 2 , O 2 ) suggests that product ions formed by dissociative charge transfer are also produced by photoionization via a similar set of excited predissociative states of CO 2 + (CO + , N 2 + , O 2 + ). By preparing CH 3 SH + , CH 3 CH 2 SH + , and CH 3 SCH 3 + in their ground states by photoionization of the corresponding neutrals, we have examined the dissociation of these ions via collision activation. Equipped with two radio frequency octopole ion guide reaction gas cells, the TQDO apparatus has allowed the identification of the isomeric structure of product ions by using the charge-transfer probing method. Strong preference is observed for C-S and C-C bond scissions, leading to the formation of CH 3 + from CH 3 SH + , CH 3 CH 2 + , and CH 2 SH + from CH 3 CH 2 SH + , and CH 3 S + from CH 3 SCH 3 + as compared to C-H and S-H bond breakages. The observation of these bond selective dissociation reactions is contrary to that found in photoionization of CH 3 SH, CH 3 CH 2 SH, and CH 3 SCH 3 and is indicative of nonstatistical behavior for collisioninduced dissociation of these organosulfur ions. An application of the radio frequency ion-guide for stateselection of O + ( 4 S, 2 D, 2 P) prepared by the dissociative charge-transfer reactions of He + (Ne + , Ar + ) + O 2 is described. The success of this method has made possible the absolute total cross section measurement of the state-selected ion-molecule reactions O + (

We have developed an efficient electron time-of-flight ͑TOF͒ selection scheme for high resolution pulsed field ionization ͑PFI͒ photoelectron ͑PFI-PE͒ measurements using monochromatized multibunch undulator synchrotron radiation at the Advanced Light Source. By employing a simple electron TOF spectrometer, we show that PFI-PEs produced by the PFI in the dark gap of a synchrotron ring period can be cleanly separated from prompt background photoelectrons. A near complete suppression of prompt electrons was achieved in PFI-PE measurements by gating the PFI-PE TOF peak, as indicated by monitoring background electron counts at the Ar(11sЈ) autoionizing Rydberg peak, which is adjacent to the Ar ϩ ( 2 P 3/2 ) PFI-PE band. The rotational-resolved PFI-PE band for H 2 ϩ (X 2 ⌺ g ϩ ,v ϩ ϭ0) measured using this electron TOF selection scheme is nearly free from residues of nearby autoionizing features, which were observed in the previous measurement by employing an electron spectrometer equipped with a hemispherical energy analyzer. This comparison indicates that the TOF PFI-PE scheme is significantly more effective in suppressing the hot-electron background. In addition to attaining a high PFI-PE transmission, a major advantage of the electron TOF scheme is that it allows the use of a smaller pulsed electric field and thus results in a higher instrumental PFI-PE resolution. We have demonstrated instrumental resolutions of 1.0 cm Ϫ1 full width at half maximum ͑FWHM͒ and 1.9 cm Ϫ1 FWHM in the PFI-PE bands for Xe ϩ ( 2 P 3/2 ) and Ar ϩ ( 2 P 3/2 ) at 12.123 and 15.760 eV, respectively. These resolutions are more than a factor 2 better than those achieved in previous synchrotron based PFI-PE studies.

Absolute state-selected cross sections for the reactions O+(4S,2D,2P)+N2→N2++O, NO++N, and N++NO (and/or N++N+O) have been measured in the center-of-mass collision energy (Ec.m.) range of 0.06–40 eV employing the differential retarding potential method and the O+(2D) and O+(2P) ion state-selection schemes we developed recently. Charge transfer is the overwhelming product channel for the O+(2D)+N2 and O+(2P)+N2 reactions. Contrary to the results of previous experiments, the charge transfer cross sections for O+(2P)+N2 are found to be 30%–100% greater than those for O+(2D)+N2. This observation suggests that N2 is an excellent quenching gas for O+(2D,2P). While the Ec.m. dependencies for the cross sections of NO+ from O+(4S)+N2 and O+(2D)+N2 are similar, exhibiting a broad maximum in the Ec.m. range of 1.5–8 eV, the cross section for NO+ from O+(2P)+N2 is found to decrease as Ec.m. is decreased. The N+ signal observed in the O+(4S)+N2 reaction is attributed to the formation of N++N+O. The pathway of O++N2→N++NO to generate N+ is strongly suggested as the major channel in the reactions of O+(2D,2P)+N2, as evidenced by the observation of N+ well below the thermochemical thresholds of O+(2D,2P)+N2→N++N+O.