This report was prepared as an account of work sponsored-by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not neoessarily state or reflect those of the United States Government or any agency thereof.-3-3/21/93 molecules: a) the study of radical kinetics, b) the use of negative ion thermochemical cycles, and c) photoionization mass spectroscopic techniques. It is essential to stress the complcmc#tarity ofthese three experimental methods; theyarcall interrelated. Our goal in this essayistodissect eachofourmethodstodescribe how themeasurements arecarried out, whatthelimitations are, andtodemonstrate by direct comparison that all givethesame bondenergies. An introduction tothese three experimental programs isnow inorder. a)Radical Kinetics Supposeone measuresthekinetics of equilibrium of a halogenatom,X, witha substrate, RH. RH + X = R +XH (I) By monitoring thetime dependenceof [X] and [R] after flash photolysis, by atomic fluorescence, and/orresonance lamp photoionization detection, one can determine the absolute rateconstants kI and k.1. These rateconstants fixtheequilibrium constant, Kequi(1), which permits one todetermine AGrxn(1), fromwhichthecnthalpy, AHrxn(1), can bc extracted. Iftheheatsof formation (AHf°(RH),AHf°(X),and AHf°(XH)) are known,AHrxn(1) permits one tofind AHf°(R)whichfixes thebond energy, BDE(R-H). b)Negative IonCycles Ionchemistry canbc usedtodeducethegasphaseacidity ofa target molecule, RH. The acidity, AHacid, isthecnthalpy for theproton abstraction reaction.

From high resolution photoionization mass spectrometric data on normal and isotopically substituted N2O it is concluded that the formation of NO+ below the à state of N2O+ but above the thermochemical threshold involves the calculated but hitherto unobserved 4A″ state. This state decays to NO+ from low vibrational levels, and mainly to O+ from higher levels. The O+ ion is a more important product of N2O photodissociation than previously thought. The B̃ state of N2O+ is shown to decompose to NO+ and N2+ and the C̃ state to NO+, N2+, N+, and O+. The branching in autoionization from the Ã, B̃, and C̃ Rydberg series to states of different continua has been determined and is found to depend more on the type of series than on the principal quantum number. The shapes of the resonances are strikingly different in the different final channels. The relevance of these findings to ionospheric O++N2 reaction is discussed.

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Gaseous C 60 has been studied by photoionization mass spectrometry between the ionization threshold and 40.8 eV. An adiabatic threshold of7.57 ± 0.01 eV is observed, which may be slightly low due to hot bands. The energy derivative of the photoion yield curve is in rough agreement with the He I photoelectron spectrum on the positions of some peaks, but others are weak or absent. The discrepancy is not attributed to autoionization, but rather to selection rules governing the ejection of low energy electrons into high angular momentum waves. C6ii + is observed at higher energies, and becomes -0.6 as intense as C 6 6 at 40.8 eV. The photoion yield curve of C6ii + , approximately linear well above threshold, appears to exhibit curvature near threshold, thwarting an attempt to make a distinction between two alternative values of the second ionization potential. Fragmentation to form C s 1 is only observed at the highest energy, 40.8 eV. The unimolecular decay is modelled by quasiequilibrium theory. In this model, the kinetic shift is of the order of 30 e V, and the minimum energy for dissociation into C s 1 + C 2 seems to be 6.0-6.5 eV.

A photoionization mass spectrometric study of SiH4 at T=150 K reveals the presence of SiH+4 with an adiabatic threshold at 11.00±0.02 eV. The implications for the structure of this Jahn–Teller split state are discussed. The appearance potentials of SiH+2 and SiH+3 are 11.54±0.01 eV and ≤12.086 eV, respectively. The reaction of F atoms with SiH4 generates SiH3 (X 2A1), SiH2 (X 1A1 and a 3B1), and SiH (X 2Π) in sufficient abundance for photoionization studies. The measured adiabatic ionization potentials (eV) are: SiH3, 8.01±0.02; SiH2 (X 1A1), 9.15±0.02 or 9.02±0.02; SiH2 (a 3B1), 8.244±0.025; SiH, 7.91±0.01. The singlet–triplet splitting in SiH2 is either 0.78±0.03 or 0.91±0.03 eV. The dissociation energy of SiH is 2.98±0.03 eV. A Rydberg series is observed, converging to SiH+ (a 3Π) at 10.21±0.01 eV. Heats of formation of the various neutral and ionic species are presented, as are the stepwise bond energies of SiH4.