H (Rydberg) Atom Photofragment Translational Spectroscopy (PTS) Professor Michael N R Ashfold Dr Emma J Feltham Dr Phillip A Cook Mr Rafay H Qadiri Ms Emily E H Cottrill

ABSTRACT The fragmentation dynamics of allene and propyne molecules following photo-excitation at 203.3, and nm have been investigated by H (Rydberg) atom photofragment translational spectroscopy methods. Contrary to conclusions reached in previous photochemical studies of these molecules, at a photolysis wavelength of 193 nm, we find the translational energy spectra associated with the H atom product forming channel in both molecules to be essentially identical, and to have a form that is reproduced well by an approximate statistical model that assumes population of all possible vibrational states of the H 2 CCCH partner. Such behaviour can be most readily accommodated by assuming that, for both molecules, at the excitation energies used in the present work, internal conversion to, and isomerisation on, the ground state potential energy surface precedes fragmentation.

H ATOM PTS A powerful technique for high-resolution probing of photofragment energy states. Example: A molecule which dissociates with loss of an H or D atom. Careful choice of experimental parameters allows application of conservation of energy and momentum. E int (RA) can be determined from Time Of Flight (TOF) of the H atom from point of dissociation to a fixed detector. High resolution achieved by use of Rydberg tagging for H atom detection.

RYDBERG TAGGING Rydberg orbitals are diffuse, non- bonding orbitals with principal quantum number, n, greater than the occupied valence orbitals. Nascent H-atoms tagged at source by two-photon excitation to a Rydberg state. Rydberg states are neutral and can be stable for ~10 -3 s. H atoms which recoil along detector axis are ionised immediately prior to detection. No space-charge blurring in dissociation region gives very high resolution.

LASER SCHEMATIC

EXPERIMENTAL SCHEME Jet-cooled supersonic molecular beam of target gas seeded in Ar. Dissociation instigated by photolysis laser. H atoms tagged 10 ns afterwards by Lyman- and Rydberg beams. Recoil anisotropy and photolysis laser wavelength and power dependence can be studied. TOF spectrum of H atoms is recorded.

TKER = Total Kinetic Energy Release m H = mass of hydrogen m P = mass of partner fragment d = Time Of Flight length t = time of arrival = TOF MEASUREMENTS & DATA ANALYSIS Measured TOF of H atoms is converted to TKER of photofragments. TOF length determined from measurement of H atoms resulting from much studied UV photolysis of H 2 S. Assume mass of partner fragment. Only detect dissociation pathways forming H. Infer information about internal energy of unobserved partner fragments. h = D 0 + TKER + E int (P)

ACETYLENE Photolysis, and subsequent chemistry of C 2 H radicals, important in establishing hydrocarbon balance in atmospheres of outer planets and their moons. C 2 H 2 and C 2 H identified as important intermediates in combustion processes, and implicated in soot formation and some chemical vapour deposition (CVD) environments. This example is included to illustrate the exquisite energy resolution of the H (Rydberg) tagging method.

ACETYLENE EXPERIMENTS Mordaunt et al., JCP 108, 519 (1998). Photofragment translational spectroscopy. Direct observation of H + C 2 H (X) products. D 0 (H-CCH) = ± 8 cm -1. ~ 600 cm -1 exit channel barrier. 2 (bending) excitation in C 2 H products. Product state dependent recoil anisotropy (cf. high and low N). Rationalise in terms of S 1 T n inter- system crossing (ISC) and eventual dissociation.

ALLENE & PROPYNE Isomeric forms of C 3 H 4 (along with cyclopropene). Important in combustion processes. Present in interstellar clouds and in the atmospheres of the outer planets. Previous Experimental Studies Photodissociation via elimination of H or H 2. Allene – few studies; suggestions that dissociation proceeds via internal conversion (IC) to ground (S 0 ) state. Propyne – at 193 nm suggested to photodissociate via loss of acetylenic rather than methyl H (from studies using isotopomers of propyne – DCCCH 3 or HCCCD 3 ).

C 3 H 4 FRAGMENTATION CHANNELS

ALLENE SPECTRA H 2 CCCH nm H + C 3 H 3 (1) (a) H atom TOF spectra resulting from photolysis of jet-cooled allene molecules at nm using incident pulse energies of 120 J (red) and 240 J (pink) with, in both cases, = 90. (b) The corresponding TKER spectra: the dashed vertical line at high TKER indicates the maximum kinetic energy release possible assuming that all observed H atoms arise via primary fragmentation process (1), while the smooth curve superimposed on the spectrum obtained at lower photolysis energies shows the energy disposal in the H + C 3 H 3 products if we assume a crude statistical model for the fragmentation of highly vibrationally excited ground state C 3 H 4 molecules.

PROPYNE SPECTRA H 3 CCCH nm H + H 2 CCCH (2) H + H 3 CCC (3) (a) H atom TOF spectrum resulting from photolysis of jet-cooled propyne molecules at nm using an incident pulse energy of 150 J, with set at 90. (b) The corresponding TKER spectrum: the dashed vertical lines at high TKER indicate the maximum kinetic energy release possible if it is assumed that all of the observed H atoms arise via primary fragmentation processes (2) and (3), (labelled TKER max (2) and TKER max (3)), respectively, the forms of the TKER spectra predicted using the statistical model and assuming fragmentation via the two channels (2) and (3) are shown beneath (in black and blue, respectively).

ALLENE & PROPYNE RESULTS TOF spectra show seemingly similar H atom distributions. Characterised by steep rise in faster H atom count, with a tail to longer time. Most H atoms have low kinetic energy, therefore partner fragments have high internal energy. Lack of resolved structure implies population of large number of rovibrational states in partner fragments. H atom profile shows no angular dependence. Statistical model for allene gives good agreement with experiment. Statistical model for propyne assuming loss of acetylenic H atom gives poor agreement, methyl H atom gives better agreement with experimental data. Secondary photodissociation evident in both TKER spectra.

PROPYNE: POSSIBLE RATIONALE Higher energy 193 nm photon allows molecule to overcome exit channel barrier on the excited state PES; dissociation occurs on this S 1 PES nm photon provides insufficient energy to overcome this barrier. Slower IC process represents predominant decay process.

CONCLUSIONS Allene Statistical dissociation of hot ground state C 3 H 4 molecule yielding H + C 3 H 3. Propyne 193 nm: could be reconciled in terms of direct dissociation on excited state PES. Further experiments needed nm: direct pathway blocked by exit barrier, IC becomes most effective decay pathway, followed by statistical dissociation of internally hot C 3 H 4 molecule, to yield a TKER spectrum very similar to that from the corresponding dissociation of allene. Implies allene propyne isomerisation rates for highly vibrationally excited ground state C 3 H 4 molecules are fast compared to their unimolecular decay. Some contamination by secondary photodissociation in all cases, most obviously for propyne where parent is very small.