Paul Regan - Academia.edu (original) (raw)
Papers by Paul Regan
The Journal of Physical Chemistry B, 2002
In the work presented here, we used photofragment translational spectroscopy and H atom Rydberg t... more In the work presented here, we used photofragment translational spectroscopy and H atom Rydberg timeof-flight (HRTOF) spectroscopy to study the primary photofragmentation channels of allyl iodide excited at 193 nm and the ensuing dissociation of the nascent allyl radicals as a function of their internal energy. Two C-I bond fission channels were found to produce the allyl radical, one channel forming I(2 P 3/2) and the other forming I(2 P 1/2). The nascent allyl radicals are dispersed as a function of the translational energy imparted from the photolysis and therefore by their internal energy. Although all of the I(2 P 3/2) and a portion of the I(2 P 1/2) channel allyl radical products have enough internal energy to overcome the 60 kcal/mol barrier to form allene + H, the data showed that a substantial fraction of the allyl radicals from the I(2 P 1/2) channel that formed with internal energies as high as 15 kcal/mol above the 60 kcal/mol barrier were stable to H atom loss. The stability is due to centrifugal effects caused by significant rotational energy imparted to the allyl radical during photolysis and the small impact parameter and reduced mass characterizing the loss of an H atom from an allyl radical to form allene + H. A photoionization efficiency (PIE) curve identified the major C 3 H 4 secondary dissociation products as allene. Comparison of the mass 40 signal in the TOF spectra at two photoionization energies showed that branching to H + propyne does not occur at near-threshold internal energies, indicating that the experimentally determined allyl f 2-propenyl radical isomerization barrier, which is lower than recent ab initio calculations of the barrier by ∼15 kcal/mol, is far too low.
Chemical Physics Letters, 2001
The application of the velocity map imaging technique to study the vacuum ultraviolet photodissoc... more The application of the velocity map imaging technique to study the vacuum ultraviolet photodissociation of molecules to give ion pair products can be a powerful means of probing ion energetics and spectroscopy. This approach, in many ways analogous to photoelectron spectroscopy, has the advantage that it may be applied to systems for which stable neutral precursors do not exist or may be difficult to prepare. In this Communication we describe the general technique and show the first demonstration of the method: CH 3 Cl ~ CH3 + + cr. The results provide insight into the vibrational frequencies of the methyl cation and the ion pair dissociation dynamics of methyl chloride, and illustrate some of the advantages of the method.
Laser Techniques for State-Selected and State-to-State Chemistry IV, 1998
The near UV photodissociation of hydrogen bromide and hydrogen iodide has been investigated using... more The near UV photodissociation of hydrogen bromide and hydrogen iodide has been investigated using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for ground and spin-orbit excited halogen atom formation have been measured as a function of excitation wavelength, as have the angular distributions for both fragmentation channels in both hydrogen halides. In the case of HI, the branching ratio measurements - which show a marked excitation wavelength dependence - reinforce previous findings at excitation wavelengths (lambda) < 250 nm, but at longer wavelengths show larger relative yields of spin- orbit excited iodine (I*) atoms than reported hitherto. The perpendicular nature of the electronic transition yielding ground state H + I products is confirmed but, again in contradiction to previous results, the angular distribution measurements show that the I* atoms arise almost entirely via a parallel photoexcitation pathway. HBr photolysis in the wavelength range 200 < (lambda) < 253 nm yields Br*/Br spin-orbit branching ratios consistently approximately 0.22; as in HI, all ground state products formed in this wavelength range derive from a perpendicular photoexcitation process, but the angular anisotropy of the fragmentation yielding excited Br* atoms shows a marked wavelength dependence. The observations are reviewed in the light of current knowledge of the repulsive excited states contributing to the near UV absorption spectra of the hydrogen halides.
Chemical Physics Letters, 2000
Physical Chemistry Chemical Physics, 1999
The ultraviolet photodissociation of jet-cooled HCl molecules at 5 wavelengths in the range 201–2... more The ultraviolet photodissociation of jet-cooled HCl molecules at 5 wavelengths in the range 201–210 nm has been investigated. Ground state hydrogen photofragments, H(S), were detected using the H Rydberg atom time-of-flight (HRTOF) technique to obtain directly the relative ...
Physical Chemistry Chemical Physics, 2000
... Paul M. Regan, Daniela Ascenzi, Eckart Wrede§, Phillip A. Cook, Michael NR Ashfold and Andr... more ... Paul M. Regan, Daniela Ascenzi, Eckart Wrede§, Phillip A. Cook, Michael NR Ashfold and Andrew J. Orr-Ewing*. School of Chemistry, Uni ersity ... The labels of bands (17) correspond with those in Table 2, and the combs above the spectra identify the spectroscopic transitions ...
Chemical Physics Letters, 1999
The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength... more The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength tail of the first UV Ž 1 1 q. absorption band A P § X S has been investigated at five wavelengths in the range 200-220 nm. Ground state 1 Ž 2 .) Ž 2. Ž. Cl P and spin-orbit excited Cl P photofragments were monitored using 2 q 1 resonance enhanced multiphoton 3r2 1 r2 ionization in a time-of-flight mass spectrometer. The product branching fractions are reported and compared with previous experimental results and high-level quantum mechanical calculations for HCl and DCl. A significant HrD isotope effect in the branching fractions is found at all the studied wavelengths, in quantitative agreement with recent theoretical predictions.
Chemical Physics Letters, 1999
The UV photodissociation dynamics of HI has been studied at 5 photolysis wavelengths, l s 222, 24... more The UV photodissociation dynamics of HI has been studied at 5 photolysis wavelengths, l s 222, 248, 258, 266 and 280 Ž 2. Ž. nm, by measuring branching fractions for formation of spin-orbit excited I P atoms using 2 q 1 resonance enhanced 1r2 Ž. Ž 2. Ž 2. multi-photon ionization REMPI detection of I P and I P. Branching fractions are derived from experimental 3r2 1 r2 REMPI signals using an independently established calibration factor, and the results are compared with previous determinations based on different H-atom detection schemes. Discrepancies between these previous measurements for photolysis wavelengths longer than 220 nm are resolved by comparison with the I-atom REMPI results and by detailed Ž 2. modelling of the H-atom detection experiments previously performed in this laboratory. The branching fraction for I P 1r2 peaks at a value of 0.50 " 0.05 for wavelengths around 250 nm.
Chemical Physics, 1998
The near ultraviolet photodissociation of hydrogen iodide has been investigated in detail using t... more The near ultraviolet photodissociation of hydrogen iodide has been investigated in detail using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for forming ground I(2P3/2) and spin-orbit excited I(2P1/2) atoms have been determined at 24 different wavelengths in the range 201.0–303.0 nm, whilst detailed angular distributions for both fragmentation channels have been measured at five wavelengths within
The Journal of chemical …, 1999
The ultraviolet photodissociation dynamics of hydrogen bromide. [The Journal of Chemical Physics ... more The ultraviolet photodissociation dynamics of hydrogen bromide. [The Journal of Chemical Physics 110, 281 (1999)]. Paul M. Regan, Stephen R. Langford, Andrew J. Orr-Ewing, Michael NR Ashfold. Abstract. The technique of ...
The Journal of …
Experimental and theoretical methods have been applied to investigate the effect of internal pare... more Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl molecules. Jet-cooled molecules within a time-of-flight mass spectrometer were prepared by infra-red ...
The Journal of Physical Chemistry B, 2002
In the work presented here, we used photofragment translational spectroscopy and H atom Rydberg t... more In the work presented here, we used photofragment translational spectroscopy and H atom Rydberg timeof-flight (HRTOF) spectroscopy to study the primary photofragmentation channels of allyl iodide excited at 193 nm and the ensuing dissociation of the nascent allyl radicals as a function of their internal energy. Two C-I bond fission channels were found to produce the allyl radical, one channel forming I(2 P 3/2) and the other forming I(2 P 1/2). The nascent allyl radicals are dispersed as a function of the translational energy imparted from the photolysis and therefore by their internal energy. Although all of the I(2 P 3/2) and a portion of the I(2 P 1/2) channel allyl radical products have enough internal energy to overcome the 60 kcal/mol barrier to form allene + H, the data showed that a substantial fraction of the allyl radicals from the I(2 P 1/2) channel that formed with internal energies as high as 15 kcal/mol above the 60 kcal/mol barrier were stable to H atom loss. The stability is due to centrifugal effects caused by significant rotational energy imparted to the allyl radical during photolysis and the small impact parameter and reduced mass characterizing the loss of an H atom from an allyl radical to form allene + H. A photoionization efficiency (PIE) curve identified the major C 3 H 4 secondary dissociation products as allene. Comparison of the mass 40 signal in the TOF spectra at two photoionization energies showed that branching to H + propyne does not occur at near-threshold internal energies, indicating that the experimentally determined allyl f 2-propenyl radical isomerization barrier, which is lower than recent ab initio calculations of the barrier by ∼15 kcal/mol, is far too low.
Chemical Physics Letters, 2001
The application of the velocity map imaging technique to study the vacuum ultraviolet photodissoc... more The application of the velocity map imaging technique to study the vacuum ultraviolet photodissociation of molecules to give ion pair products can be a powerful means of probing ion energetics and spectroscopy. This approach, in many ways analogous to photoelectron spectroscopy, has the advantage that it may be applied to systems for which stable neutral precursors do not exist or may be difficult to prepare. In this Communication we describe the general technique and show the first demonstration of the method: CH 3 Cl ~ CH3 + + cr. The results provide insight into the vibrational frequencies of the methyl cation and the ion pair dissociation dynamics of methyl chloride, and illustrate some of the advantages of the method.
Laser Techniques for State-Selected and State-to-State Chemistry IV, 1998
The near UV photodissociation of hydrogen bromide and hydrogen iodide has been investigated using... more The near UV photodissociation of hydrogen bromide and hydrogen iodide has been investigated using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for ground and spin-orbit excited halogen atom formation have been measured as a function of excitation wavelength, as have the angular distributions for both fragmentation channels in both hydrogen halides. In the case of HI, the branching ratio measurements - which show a marked excitation wavelength dependence - reinforce previous findings at excitation wavelengths (lambda) < 250 nm, but at longer wavelengths show larger relative yields of spin- orbit excited iodine (I*) atoms than reported hitherto. The perpendicular nature of the electronic transition yielding ground state H + I products is confirmed but, again in contradiction to previous results, the angular distribution measurements show that the I* atoms arise almost entirely via a parallel photoexcitation pathway. HBr photolysis in the wavelength range 200 < (lambda) < 253 nm yields Br*/Br spin-orbit branching ratios consistently approximately 0.22; as in HI, all ground state products formed in this wavelength range derive from a perpendicular photoexcitation process, but the angular anisotropy of the fragmentation yielding excited Br* atoms shows a marked wavelength dependence. The observations are reviewed in the light of current knowledge of the repulsive excited states contributing to the near UV absorption spectra of the hydrogen halides.
Chemical Physics Letters, 2000
Physical Chemistry Chemical Physics, 1999
The ultraviolet photodissociation of jet-cooled HCl molecules at 5 wavelengths in the range 201–2... more The ultraviolet photodissociation of jet-cooled HCl molecules at 5 wavelengths in the range 201–210 nm has been investigated. Ground state hydrogen photofragments, H(S), were detected using the H Rydberg atom time-of-flight (HRTOF) technique to obtain directly the relative ...
Physical Chemistry Chemical Physics, 2000
... Paul M. Regan, Daniela Ascenzi, Eckart Wrede§, Phillip A. Cook, Michael NR Ashfold and Andr... more ... Paul M. Regan, Daniela Ascenzi, Eckart Wrede§, Phillip A. Cook, Michael NR Ashfold and Andrew J. Orr-Ewing*. School of Chemistry, Uni ersity ... The labels of bands (17) correspond with those in Table 2, and the combs above the spectra identify the spectroscopic transitions ...
Chemical Physics Letters, 1999
The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength... more The photodissociation of jet-cooled DCl molecules subsequent to excitation in the long-wavelength tail of the first UV Ž 1 1 q. absorption band A P § X S has been investigated at five wavelengths in the range 200-220 nm. Ground state 1 Ž 2 .) Ž 2. Ž. Cl P and spin-orbit excited Cl P photofragments were monitored using 2 q 1 resonance enhanced multiphoton 3r2 1 r2 ionization in a time-of-flight mass spectrometer. The product branching fractions are reported and compared with previous experimental results and high-level quantum mechanical calculations for HCl and DCl. A significant HrD isotope effect in the branching fractions is found at all the studied wavelengths, in quantitative agreement with recent theoretical predictions.
Chemical Physics Letters, 1999
The UV photodissociation dynamics of HI has been studied at 5 photolysis wavelengths, l s 222, 24... more The UV photodissociation dynamics of HI has been studied at 5 photolysis wavelengths, l s 222, 248, 258, 266 and 280 Ž 2. Ž. nm, by measuring branching fractions for formation of spin-orbit excited I P atoms using 2 q 1 resonance enhanced 1r2 Ž. Ž 2. Ž 2. multi-photon ionization REMPI detection of I P and I P. Branching fractions are derived from experimental 3r2 1 r2 REMPI signals using an independently established calibration factor, and the results are compared with previous determinations based on different H-atom detection schemes. Discrepancies between these previous measurements for photolysis wavelengths longer than 220 nm are resolved by comparison with the I-atom REMPI results and by detailed Ž 2. modelling of the H-atom detection experiments previously performed in this laboratory. The branching fraction for I P 1r2 peaks at a value of 0.50 " 0.05 for wavelengths around 250 nm.
Chemical Physics, 1998
The near ultraviolet photodissociation of hydrogen iodide has been investigated in detail using t... more The near ultraviolet photodissociation of hydrogen iodide has been investigated in detail using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for forming ground I(2P3/2) and spin-orbit excited I(2P1/2) atoms have been determined at 24 different wavelengths in the range 201.0–303.0 nm, whilst detailed angular distributions for both fragmentation channels have been measured at five wavelengths within
The Journal of chemical …, 1999
The ultraviolet photodissociation dynamics of hydrogen bromide. [The Journal of Chemical Physics ... more The ultraviolet photodissociation dynamics of hydrogen bromide. [The Journal of Chemical Physics 110, 281 (1999)]. Paul M. Regan, Stephen R. Langford, Andrew J. Orr-Ewing, Michael NR Ashfold. Abstract. The technique of ...
The Journal of …
Experimental and theoretical methods have been applied to investigate the effect of internal pare... more Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl molecules. Jet-cooled molecules within a time-of-flight mass spectrometer were prepared by infra-red ...