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High resolution H (Rydberg) atom photofragment translational spectroscopy studies of the photodissociation of small organic molecules

H atom Rydberg tagging is a high resolution time of flight (TOF) method useful for studying the photodissociation of H atoms from molecular and radical species. High resolution is achieved due to the absence of space charge blurring effects and electron recoil associated with ionisation detection methods and the relatively long flight distances and times associated with field free time of flight techniques. By observation of the H atom TOF profile, a kinetic energy release spectrum can be determined, revealing the distribution of quantum states amongst the partner products of the dissociation event. Anisotropy information is available by the comparison of photodissociation events under different polarisations of the dissociation laser field. Tagging of the H atoms to a high lying quantum state (n=30-90) is achieved with a two-colour double resonant excitation scheme. Ground state H atoms are promoted to n=2 by single photon Lyman-a (121.6 nm) excitation and subsequently to a high Rydberg state by a 366 nm photon. Field ionisation of the H atoms takes place close to the detector. The experimental scheme is shown below:

Fig. 4: H-atom Rydberg tagging experimental scheme

Pyrrole:

The fragmentation dynamics of pyrrole molecules following excitation at many wavelengths in the range 193.3 < l_phot < 254.0 nm have been investigated by H Rydberg atom photofragment translational spectroscopy.

Fig. 5: Photodissociation of the N-H bond in Pyrrole

Excitation at the longer wavelengths within this range results in population of the ¹A₂ excited state but, once l_phot 225 nm, the electric dipole allowed ¹B₂ ¬ X¹A₁ transition becomes the dominant absorption. (See Fig. 7)

Fig. 6: TKER spectrum showing direct dissociation of electronically excited pyrrole (fast, structured peak) from 244 nm and 220 nm photolysis and statistical decay of hot ground state pyrrole (slow, unstructured peak) following excitation at 220 nm.

All of the total kinetic energy release (TKER) spectra show a fast peak, centred at TKER 7000 cm^-1 as shown in Fig. 6. Structure is evident in this peak, particularly in spectra recorded at the longer excitation wavelengths, and reveals selective population of vibrational levels of the pyrrolyl radical co-fragment. These have been assigned by comparison with calculated normal mode vibrational frequencies, leading to a precise determination of the N–H bond strength in pyrrole. The recoil anisotropy of the fast H atom photofragments is seen to depend upon the vibrational level of the pyrrolyl co-fragment. A second, slow peak is evident in spectra recorded at shorter photolysis wavelengths (evident in Fig. 6), and eventually becomes the dominant feature. This slow peak exhibits no recoil anisotropy indicating statistical decay of highly vibrationally excited ground state molecules. There are two possible paths for the formation of highly vibrationally excited ground state molecules from the ¹B₂ state (Fig.7, blue curve), either a direct radiationless coupling or an alternative non-adiabatic route via the ¹A₂ state responsible for the direct fragmentation dynamics (shown in Fig. 7 in red). Decay from the vibrationally excited ground state yields a statistical distribution of slow H atoms mirroring the density of states in the vibrationally excited co-fragments (probably mostly cyano-allyl).

Fig. 7: Calculated energetics for pyrrole dissocation to pyrrolyl and alternative routes to other fragments (cyano-allyl and HCN). Photoexcitation occurs vibronically from the X¹A₁ state (green) to the ¹A₂ state (red) or as a fully allowed transition to the ¹B₂ state (blue).

Allene and Propyne

The fragmentation dynamics of allene and propyne molecules following photo-excitation at 193.3 nm and at 121.6 nm have also been investigated by H(D) Rydberg atom photofragment translational spectroscopy (PTS). Non-adiabatic processes are seen to play a dominant role in the fragmentation of both molecules at both wavelengths. The total kinetic energy release (TKER) spectra of the H (and D) atoms resulting from H₂CCCH₂, H₃CCCH and D₃CCCH photolysis at 193.3 nm are found to be essentially identical. This finding contradicts conclusions reached in a number of earlier studies of propyne photochemistry at this wavelength. The observed energy disposal, and the isomer independence, is most readily rationalized by assuming that the fragmentation of both molecules following excitation at 193.3 nm is preceded by internal conversion to the ground (S0) state potential energy surface, and that the isomerisation rate of the resulting highly vibrationally excited S0 molecules is faster than their unimolecular decay rate. The time-of-flight (TOF) and TKER spectra of the H and D atoms resulting from 121.6 nm photolysis of allene, propyne and propyne-d₃ show significant differences, however. The differences can be reconciled by assuming that there are two competing pathways for forming H(D) atoms following 121.6 nm excitation of propyne. One, involving selective cleavage of the acetylenic H₃CCCH bond, is assumed to occur from the excited electronic state prepared by photon absorption. The other, which yields a slower distribution of H(D) atoms, is presumed to arise as a result of radiationless transfer to a lower electronic state, isomerisation and subsequent unimolecular decay. The TOF and TKER spectra of the H atoms resulting from 121.6 nm photolysis of allene are found to be indistinguishable from those associated with this second, ‘statistical’ fragmentation channel in propyne.

 

Recent publications:

Bríd Cronin, Michael G.D. Nix, Rafay H. Qadiri and Michael N.R. Ashfold,

High resolution photofragment translational spectroscopy studies of the near ultraviolet photodissociation of pyrrole.
Phys. Chem. Chem. Phys., accepted and online

R.H. Qadiri, E.J. Feltham, N.H. Nahler, R. Pèrez García and M.N.R. Ashfold,

Propyne and allene photolysis at 193.3 nm and at 121.6 nm.
J. Chem. Phys. 119:12842, 2003 (pdf)