The photodissociation spectroscopy of MgCH 4 ϩ has been studied in a reflectron time-of-flight mass spectrometer. MgCH 4 ϩ molecular absorption bands are observed to the red of the Mg ϩ (3 2 P J ←3 2 S 1/2 ) atomic ion resonance lines. The photofragmentation action spectrum consists of a broad structureless continuum ranging from 310 nm to 342 nm, and peaking near 325 nm. In this spectral region, both the nonreactive ͑Mg ϩ ͒, and two reactive fragmentation products ͑MgH ϩ and MgCH 3 ϩ ͒ are observed, all with similar action spectra. The product branching is independent of wavelength, Mg ϩ :MgCH 3 ϩ :MgH ϩ ϳ60:33:7. The absorption is assigned to the transition (1 2 E←1 2 A 1 ) in C 3v symmetry ͑with 3 coordination͒, followed by a geometrical relaxation of the complex toward states of 2 B 1 and 2 B 2 symmetry in C 2v geometry ͑with 2 coordination͒.Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg ϩ product angular distribution to be isotropic, with an average translational energy release which increases slightly from E t ϳ370Ϯ150 cm Ϫ1 at 332.5 nm to E t ϳ520Ϯ180 cm Ϫ1 at 315 nm. These values are less than 2% of the available energy and are well below statistical expectations. Analogous experiments on MgCD 4 ϩ show the kinetic energy release in the nonreactive channel to be significantly larger for the CD 4 case, ranging from E t ϳ540Ϯ180 cm Ϫ1 at 332.5 nm to E t ϳ830Ϯ200 cm Ϫ1 . These results clearly demonstrate that the dissociation is nonstatistical. Preliminary ab initio potential surface calculations suggest a possible dynamical mechanism to explain these unusual results.

We have studied the structure and dissociation dynamics of the weakly bound bimolecular complex Al+(C2H4) by photodissociation spectroscopy in the 216–320 nm spectral region. Experimental studies are supported by ab initio electronic structure calculations of the ground and low-lying excited states of the complex. Al+ is the dominant photofragment observed throughout the absorption profile. C2H4+ charge transfer product is also observed for shorter photolysis wavelengths, λ<252 nm. The Al+–C2H4 bond dissociation energy is measured as D0=0.37±0.15 eV. Three molecular absorption bands are observed and assigned to the transitions (2 1A1,1 1B1,1 1B2←1 1A1) in C2v equilibrium complex geometry. The excited states are of predominantly charge-transfer character correlating with the product channel Al(3s23p)+(C2H4)+. The 2 1A1 and 1 1B2←1 1A1 absorption bands appear broad and structureless. This observation is consistent with ab initio results that suggest a pathway for rapid nonadiabatic dissociation through a 1 1B2–1 1A1 surface crossing facilitated by a stretch in the C–C bond of ethylene. In contrast the 1 1B1←1 1A1 molecular band shows significant vibrational structure. Spectroscopic analysis yields a band origin (000=40 042 cm−1) and corresponding vibrational mode frequencies for the 1 1B1 excited state. The observed modes have been assigned to the intermolecular Al–C2H4 stretch of a1 symmetry (ν2=230 cm−1), the Al–C2H4 out-of-plane wag with b1-symmetry (ν3=328 cm−1), and two intramolecular ethylene modes of a1 symmetry at 1264 and 1521 cm−1. The assignment for these higher frequency ethylene modes is not conclusive.

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We have studied the photodissociation spectroscopy of weakly bound Al + salkene bimolecular complexes (sethene, spropene, and sbutene) in the 216-320 nm spectral region. Molecular absorption bands are assigned to photoinduced charge transfer transitions to states that correlate with the Al(3s 2 3p) + (alkene) + product channels. Similarities in the absorption spectra for Al + sethene, spropene, and s1-butene suggest similar equilibrium geometries with the metal ion lying above the alkene CdC π-bond in each case. The vibrational resonance structure is assigned to intramolecular modes of the alkene. The absorption spectra for Al + s2-butene are broader with less identifiable structure. A clear threshold for charge transfer dissociation to each alkene ion product gives a limit for the corresponding Al + salkene bond dissociation energy: D 0 ′′-(AlsX) e 9.2, 19.1, 19.8, 24.7, and 28.2 kcal/mol for X ) ethene, propene, 1-butene, cis-2-butene, and trans-2-butene, respectively. Experimental results are in good agreement with ab initio predictions.

We have studied the dissociation dynamics of Mg2CH4+ ion–molecule clusters through mass-resolved photodissociation spectroscopy, coupled with translational energy spectroscopy. We have observed distinct molecular absorption bands in the red (690–615 nm) and green (580–545 nm) spectral regions. Mg+ is the dominant fragmentation product in each band. We observe a significant energy release into relative translation and a pronounced photofragment anisotropy (β>0), consistent with a rapid dissociation. Based on the observed anisotropy and the result of an ab initio structure calculation, we assign the red band to the parallel transition 2 2A′←1 2A′ and the green band to a combination of 1 2A′′←1 2A′ and 3 2A′←1 2A′ transitions, all in Cs symmetry. These results are compared with earlier results from the photodissociation spectroscopy of the more strongly bound Mg2CO2+ and Mg2H2O+ bimolecular complexes.