H Rydberg Atom Photofragment Translational Spectroscopy
H Rydberg atom Photofragment Translational Spectroscopy (PTS) is an experimental, laser based, technique pioneered in Bielefeld, Germany and in our own group in Bristol. The technique is used to study gas phase molecular photodissociation processes which lead to the production of a hydrogen atom as one of the photo-products. These processes form an integral part of the complex chemistries of planetary atmospheres, combustion, plasmas, etc. The aim of the research presented here is to measure the detailed information needed to understand the mechanisms which drive molecular photodissociations.
The principle of the PTS technique involves measuring the time of flight (velocity) of the H atom products, and using conservation of energy, and
momentum, to convert this to the kinetic energy release of the photofragments. The observed spread of (kinetic) energies reflects the internal energy (and
angular) distribution of the unobserved partner moiety. As will be shown later, the use of "Rydberg tagging" of the H atom photofragments
leads to high kinetic energy resolution in the resulting data, and thus a more easily understood account of the photodissociation dynamics.
The potential of this method is best visualised by reference to an example. Vacuum ultraviolet photo-excitation of methane leads to fragmentation. What products
are formed, and with what quantum yields? We were able to provide the first definitive answers to these questions by measuring the times-of-flight, and
thus the kinetic energies, of the resulting H atom photofragments. The method relies on energy conservation: H atoms formed in association with internally
excited partner fragments (e.g. vibrationally excited CH3 radicals) must possess less kinetic energy. The H atom kinetic energy spectrum thus carries an imprint of
the population distribution of the (unobserved) partner fragment - precisely the information sought! Other molecules whose primary photochemistry we have
studied in this way include the hydrogen halides, HCN, H2O, H2S, NH3, PH3, C2H2,
CH3SH, CH3NH2, HCOOH, HCHO, HFCO, HN3, allene (CH2CCH2), propyne (CH3CCH),
ketene (H2CCO) and the CH3 radical. Current activities are centred on the photolysis of small heteroaromatic molecules, e.g. substituted pyrroles and phenols.
Examples of recent work can be found in our publications list.
Many techniques used for studying photodissociation processes rely on the detection of ions which are produced either directly, or by ionisation
of neutral photofragments in the volume where photodissociation occurs (the interaction region). The formation of ions leads to a high detection
efficiency, as the use of correctly tuned electric fields (ion optics) may be used to accelerate them toward a detector. However, the formation of
many charged species in the small focal volume of the interaction region of the experiment (in this case the focus of a laser beam) can lead to
blurring of the resulting data due to Coulomb repulsion forces. In order to circumvent this problem, the photofragments can be initially excited
into certain high energy electronic (Rydberg) neutral states that have long (ms) lifetimes and are just below (in energy) an Ionisation Potential.
These "Rydberg tagged", but still neutral photofragments can then move with unperturbed velocities to the ion detector where a small
electric field is present and (only) ionises the tagged photofragments immediately prior to detection.
Experimental Details
The molecule under consideration (which is generally seeded to ~ 5 - 10% concentration in Ar as a carrier) is introduced through a pulsed nozzle assembly into a differentially pumped (~ 10-6 Torr) high vacuum chamber. The expanding gas pulse is then 'skimmed' by means of a small aperture (~ 1 mm diameter) directly in front of the nozzle assembly. The resulting highly collimated gas beam which enters the main chamber (pressure ~ 10-7 Torr) has low internal energy, and a well defined velocity distribution.
In the main chamber the gas beam is first crossed by the focused photolysis laser radiation, which is provided by a Nd:YAG pumped dye laser assembly. By means of frequency doubling and tripling optics it is possible to work in the wavelength range l ~ 200 - 300 nm, with output energies up to 2-5 mJ per laser pulse.
A Fresnel rhomb assembly, placed before the focusing lens, allows us to set the polarisation of this laser, and thus probe angular dependencies in the photolysis event. Any unwanted ions formed in the photodissociation and/or detection process are immediately extracted from the interaction region by means of a small electric field.
After a short time delay (~ 10 ns) any H atoms formed from the photolysis event are 'tagged' by a two colour, two photon excitation process to a Rydberg state of high principal quantum number (n ~ 80). This scheme involves first exciting the H atoms from their electronic ground state (in which, on purely energetic grounds, they must be formed), to a state with n = 2. The required Lyman-a radiation to promote this excitation is provided by a Nd:YAG pumped dye laser operating at l = 364.5 nm. This is then frequency tripled in a gas cell containing a Kr/Ar mix to produce the required 121.6 nm (see below). A third laser assembly (temporally and spatially overlapped with the Lyman-a radiation) provides photons (l ~ 365 nm) which further excite the H atoms to a state with n ~ 80. Any H atoms so tagged recoil from the photolysis region, and a small fraction will fly up the time of flight axis (perpendicular to the page) where they are subsequently ionised, and detected.
Efficient generation of the Lyman-a radiation is performed using the stainless steel cell shown schematically above. This cell is sealed at both ends with a MgF2 lens attached to the main chamber and a quartz window. A pre-determined mixture of clean Kr and Ar gas is introduced into the cell and is continuously circulated using the convection currents generated from the dry ice/methanol slush bath indicated above. By optimising ('phase-matching') the Ar/Kr mixture, the efficiency of generating the Lyman- a radiation is increased ~ 100 fold compared to just using a low pressure of pure Kr. The continuous circulation of the mixture ensures efficient VUV generation for up to 2 months.
Our recent experimental studies have focussed on the photodissociation of heteroaromatic molecules like phenol, following excitation over a wide range of wavelengths, from 280 to 210 nm. Excitation at the longer wavelengths in this range prepares phenol in its optically bright, bound excited S1 (ππ*) singlet state. Theoretical studies by Sobolewski et al1, has predicted the presence of an optically dark, dissociative S2 (πσ*) potential that crosses through the potential energy surfaces of the S1 state and that of the ground state. These crossing points in the O-H stretching coordinate develop into conical intersections when out of plane vibrations are taken into account. Direct O-H bond fission has thus been predicted following excitation at energies above the S1/S2 crossing. This direct dissociation would produce ground electronic state phenoxy radicals and H-atoms. The estimate of the onset of this direct dissociation is given by the energetic position of the conical intersection between the S1 and the S2 states, which is theoretically predicted to lie at 5 eV (~ 248 nm).
Highly structured H-atom spectra have been recorded following excitation of phenol in the range 244 nm > λ > 220 nm; the observed structure is interpreted in terms of S1/S2 coupling, and provides insight into the nature of the vibrations promoting this coupling. Structured H-atom PTS spectra have also been recorded at much longer wavelengths (λ ~ 275 nm). However, this must arise via two successive non-radiative processes: internal conversion from the initially prepared S1 levels to high vibrationally excited levels of S0 and subsequent S0/S2 coupling via the conical intersection at large RO-H to form the H + ground state phenoxy radical products.
1. Sobolewski et al, Physical Chemistry Chemical Physics, 4, 1093-1100 (2002)
Phenol PTS. This figure shows the H-atom PTS spectra recorded upon phenol photodissociation at three wavelengths - 244 nm in green, 236 nm in blue and 228 nm in red.
Links to other Rydberg tagging groups:
Davis Group, Cornell University