Robert T. Carter

Resonance enhanced multiphoton ionization time-of-flight study of CF2I2 photodissociation

The resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) technique was applied to study the complex dissociation of CF2I2 in a supersonic expansion. Using nanosecond excitation at 248, 266, and 304 nm and probing the I3/2 and/or I1/2 photoproducts, we determined the photofragment speed distribution, the recoil anisotropy parameter β and the branching ratio for each of the decay channels. At 248 nm the dissociation proceeds according to a concerted three-body decay, CF2I2→hvCF2+I1/2+I3/2 (90%) or CF2I2→hvCF2+I3/2+I3/2 (10%) consistent with our previous results obtained from photofragment translational spectroscopy. At 304 nm three competing decay channels were found to be operative. In decreasing order of branching, these are: two-body decay to CF2Iu2009+I1/2, sequential three-body decay CF2I2→ lim hvCF2Iu2009+u2009I3/2→CF2u2009+u2009I3/2+I3/2, and a concerted three-body decay yielding CF2u2009+I3/2+I3/2. The positive, almost maximum β values observed for all the primary steps in the 248, 266 and 304 nm photodisso...

A HIGH-RESOLUTION STUDY OF THE NO3 RADICAL PRODUCED IN A SUPERSONIC JET

The 0oo vibrational band of the B2E′ ← X2A′2 transition in the jet-cooled NO3 radical has been studied at high resolution in the frequency- and time-domains. Under these cold conditions, the absorption band exhibits vibrational and rotational structure. The fluorescence decays are consistent with the previously reported fluorescence lifetime and some show superimposed quantum beats. A two-tier coupling scheme is proposed: the B2E′ ν = 0 level is vibronically coupled to vibrational levels in the A and/or X state, which are in turn coupled to ground state levels. Estimates of the pertinent coupling matrix elements and state densities are given.

Vector correlations in the photodissociation of OClO Ã 2A2(ν1, 0, 0)

Abstract Using resonant-enhanced multiphoton ionization combined with the time-of-flight method, the photodissociation OClO(Au2008 2 A 2 )xa0→xa0ClO(Xu2008 2 Π)+O( 3 P) was investigated at two wavelengths which were found to give access to excitation below (397.5 nm) and above (359.7 nm) the predicted exit barrier on the Au2008 2 A 2 surface. The complete angular correlations between the vectors μ , v and J for ClO( v =0) were determined at both wavelengths, providing information on the direct and indirect dissociation pathways on the Au2008 2 A 2 surface of OClO.

Quantum beat study of the nuclear hyperfine structure of OD and Ar⋅OD in their A 2Σ+ electronic states

The nuclear hyperfine structure of OD and Ar⋅OD in their Au20092Σ+ electronic states has been studied by quantum beat spectroscopy. The very cold transient species were produced in a supersonic expansion using a pulsed discharge nozzle. Coherent excitation of hyperfine (hf) states, arising from one fine structure (OD) or rotational (Ar⋅OD) level, created quantum beats on the fluorescence decay. The beat frequencies, which correspond to energy separations between hf levels, could be measured to ±75 kHz. The splitting of the hf levels into their Zeeman components was investigated in a weak magnetic field. A fit of the zero field and Zeeman data yielded the relevant constants for the nuclear magnetic and electric quadrupole hyperfine interactions as well as the pertinent g‐factors in each species. In the case of OD, the hf parameters agree well with those reported previously but are more accurately defined. For Ar⋅OD the previously unknown hyperfine and spin‐rotation parameters of the Au20092Σ+ state were determined...

Photodissociation of ClNO2 at 235 nm

Abstract The photodissociation of jet-cooled nitryl chloride ClNO2+hν→Cl+NO2 at 235 nm was investigated by resonance enhanced multiphoton ionisation/time-of-flight spectroscopy. In a one-colour experiment using nanosecond laser pulses, the Cl ground state ( 2 P3/2) photofragment was detected and its speed distribution P(v) and recoil anisotropy β were obtained. Two contributions are observed in the P(v) which are assigned to a Cl 2 P3/2 fragment emerging with a X 2 A 1 ground state and electronically excited (most likely in the A 2 B 2 state) NO2 counterfragment, respectively. The recoil anisotropy was found to be β=+1.1±0.1, yielding an upper limit of the dissociation lifetime of τ∼2 ps.

The photodissociation dynamics of nitric acid studied at 193 nm by LIF and REMPI–TOF methods

Abstract The 193 nm photodissociation of HNO 3 in a supersonic jet has been investigated by LIF and REMPI–TOF techniques probing the OH ( X 2 Π) and the O ( 3 P ) products, respectively. The measured rotational state distribution of OH is shown to be dominated by a bimodal distribution consistent with the previously reported bimodal translational energy distribution of OH. An additional decay channel, yielding O ( 3 P ) fragments with high kinetic energy, has been observed and tentatively assigned to HNO 3 +hν → O ( 3 P )+ HONO ( X 1 A ′ ) . Energetic considerations indicate that about one third of the HONO products are unstable, decaying further to OHxa0+xa0NO.

Quantum beat spectroscopy in chemistry

Quantum beat spectroscopy is a Doppler-free time domain method nbased on the creation of molecular coherences with a laser pulse and the nmeasurement of their subsequent time evolution. Fourier transformation of nthe time evolution allows a spectrum in the frequency domain with lifetime- nlimited resolution to be recovered. This article gives an account of the ntechnique applied in high resolution mode which allows the study of nisolated molecules, radicals and complexes in extensive detail and with ngreat accuracy.

QUANTUM BEAT SPECTROSCOPY OF JET-COOLED TRANSIENT RADICALS GENERATED BY A PULSED ELECTRICAL DISCHARGE

Abstract Using a pulsed electrical discharge nozzle, we have extended quantum beat spectroscopy to radicals in a molecular beam. To demonstrate the method, we studied three prototypical systems: the diatomic radical OD, the radical van der Waals complex Ar·OD and the polyatomic vinoxy radical (CH 2 CHO). In all three cases hyperfine quantum beats were observed and identified on the fluorescence decays from excited electronic states with or without an external magnetic field. The application of this method to other radical systems is discussed.

Photodissociation of OClO and Ar/OClO and H2O/OClO clusters studied by the resonance enhanced multiphoton ionization-time of flight method

The photodissociation of jet-cooled OClO following excitation into the Au200a2A2 state at around 350 nm was investigated in homogeneous OClO and large heterogeneous Ar/OClO and H2O/OClO clusters (estimated cluster size n∼800–2600) by probing the O (3P) and ClO (Xu200a2Π) photofragments using the resonance enhanced multiphoton ionization-time of flight technique. Action spectra, photofragment excitation spectra and photofragment speed distributions were recorded and compared to those for monomer dissociation. OClO was found to occupy both surface and interior sites in the heterogeneous clusters with the percentage of surface and interior dissociation processes being ∼50% for large cluster sizes. Both O and ClO photofragments generated in the cluster interior are translationally thermalized with T∼300 K and the ClO fragments are strongly rotationally and vibrationally relaxed. This is most important for vibration as monomer dissociation yields ClO containing up to 8 vibrational quanta at this photolysis wavelengt...

The rotational state dependence of the dynamics of pyrimidine in the S1 (1B1) state studied by time resolved spectroscopy

The dynamics of pyrimidine in three vibronic levels of the S1 (1B1) state with excess energies of 1100–1300 cm−1 above the 00 level has been studied by time resolved spectroscopy. The fluorescence decays from 40 to 50 single rovibronic levels in each vibronic band were measured and were found to be biexponential due to coupling with background triplet states. Analysis of the decays yielded the amplitude ratio of the fast (coherent) and slow (incoherent) component of the fluorescence intensity, A+/A−, and the lifetime of the slow component, being τu2009≊700–900 ns. Both data show a clear dependence on the S1 rotational quantum number J. The A+/A− ratio, which reflects the number of coupled states, takes the form of a linear dependence on (2J+1) implying full rovibronic state mixing in the background 3B1 state. More states were found to be coupled than could be accounted for by the calculated background state density, suggesting further state mixing, possible mechanisms for which are discussed. The lifetime res...