Photofragmentation spectroscopy of Al+(C2H4)

Abstract

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.

Photodissociation spectroscopy of Ca+(C2H4)

“…An additional state at slightly higher energies (3 2 B 2 ) correlates with an ethylene-centered excited state ͓C 2 H 4 (a 3 B 1u )͔; a similar ligand-centered excited state was also found previously in Mg ϩ ͑C 2 H 4 ͒. 6 CIS vertical excitation energies are summarized in Table I. Consistent with the asymptotic energy shifts noted above, the 1 2 B 2 , 2 2 A 1 , 1 2 A 2 , 3 2 A 1 , and 1 2 B 1 states that correlate with Ca ϩ (3d) are all too high in energy at the CIS level.…”

“…We have previously noted similar behavior involving the lowest-lying excited states of (B 2 ) character in both Mg ϩ (3p) -and Al ϩ (3p) -C 2 H 4 complexes. 6,7 In each of those cases the interactions involve the low-lying atomic 3p-orbitals rather than the 3d-orbital as is the nominally the case here. However, the potential curves of Fig.…”

“…This is in contrast to results for the analogous Mg ϩ ͑C 2 H 4 ͒ and Al ϩ ͑C 2 H 4 ͒ complexes studied previously where the 3p y (B 2 ) states are very much more strongly bound than the analogous 3 p x (B 1 ) states due to the better molecular orbital overlap in B 2 symmetry. 6,7 We have also calculated vibrational frequencies for the 4 p(2 2 B 1 and 2 2 B 2 ͒ excited states of the complex at the CIS level. As expected from the similar shape for the two states, the ab initio vibrational frequencies are similar.…”

“…[1][2][3][4][5][6] For example, we have investigated excited state chemical dynamics in weakly bound bimolecular complexes M ϩ ͑CH 4 ͒ ͑with MϭMg and Ca͒. 3,4 For Mg ϩ ͑CH 4 ͒, the broad continuum Mg ϩ (3s→3 p) absorption spectrum is consistent with a large geometry change on excitation and fast dissociation.…”

“…Here we report these results and compare them with results from recent experiments on similar Mg ϩ ͑C 2 H 4 ͒, Al ϩ ͑C 2 H 4 ͒, and Ca ϩ ͑C 2 H 2 ͒ -bonded complexes. [6][7][8] …”