Surface enhanced Raman (SER) and resonance Raman (SERR) techniques have been
used in situ to investigate thionine-modified gold electrodes. New surface roughening
procedures for gold electrodes have resulted in an order of magnitude increase in the
Raman signals. As a result of this, Raman spectra from leucothionine have been
observed for the first time. The surface Raman spectra of both thionine and leucothionine
are essentially unchanged over the pH range from 1.3 to 7 but both show major
changes at pH 10. This behaviour has been r&#232;lated to changes in the absorption spectrum
of thionine at pH 1.0 where the compound is believed to exist as thionine hydroxide. At
pH 1.3 and 7 the Raman signals from thionine arise from a combination of surface
enhancement and resonance enhancement processes, whereas signals from leucothionine
arise solely from surface enhancement. At pH 10 surface enhancement processes
give rise to Raman intensity for both thionine and leucothionine.

H. R. Virdee

R. E. Hester

Surface-Enhanced Raman
Spectroscopy of
Thionine-modified Gold
Electrodes

The role of polar solvent in determining the photophysics of normal (b*) and anomalous
(a*) fluorescence from 4-N,N-dimethylaminobenzonitrile (DMABN) and related compounds
has been investigated by means of picosecond time-resolved fluorescence studies
in non-polar/polar solvent mixtures, and by electronic spectroscopic studies on complexes
of 1:1 stoichiometry prepared by nozzle expansion. The solution phase results
indicate clearly that a DMABN-polar solvent complex is formed initially in either
ground or excited state, and that this rearranges to a geometry from which the b* to a*
state transition can occur. Specific solvent-solute interactions are clearly indicated by
this work, and these lead to the necessity of modelling b* state decay kinetics in solution
by a distribution of decay times.Laser induced fluorescence studies of jet-cooled solvated complexes of DMABN and
the related molecule 4-aminobenzonitrile (4-ABN) provide evidence for the existence of
1:1 complexes involving interaction of a protic solvent molecule with either the cyano
group or the amino group. In the case of 4-ABN both types of complex are fluorescent
whereas in the case of DMABN, those complexes involving interaction with the
dimethylamino group appear to be non-emissive. This behaviour may be due to
formation of DMABN-solvent exciplex which constitutes a dark state under jet-cooled
conditions. Neither the DMABN bare molecule nor the observed 1:1 solvated complexes
exhibit a* emission in the jet.

Anita C. Jones

David  Phillips

Laser Induced Fluorescence of
4-N,N-Dimethylaminobenzonitrile
(DMABN) and Related
Compounds

Techniques are described for obtaining the energy-dependence of state to state
rotational energy transfer (RET) cross-sections in thermal cell experiments through
velocity selection by Doppler shift. The case of polarised excitation and detection is
discussed in detail and expressions presented which enable the state multipoles of
population, orientation and alignment to be obtained from appropriate RET experiments.
It is found that the translational order present in a non-isotropic collision
environment, such as that resulting from excitation with a narrow linewidth laser, causes
state multipoles of different rank to become coupled. Experimental results are presented
for the energy dependence of cross-sections for state to state transfer of
population and of orientation in Li2-Xe collisions. The way the relative velocity vector
changes as a result of a collision may also in principle be studied using a high resolution
probe (e.g. a second laser) of the velocity spread following RET.

Christopher P. Fell

Anthony J. McCaffery

Katharine L. Reid

Anton  Ticktin

Benjamin J. Whitaker

Velocity Dependence of
Rotationally Inelastic
Cross-Sections

Infrared absorption profiles have been measured for cyclopropane and 1,1-dimethylcyclopropane
clusters by monitoring the depletion of signal intensity in a mass spectrometer
as a function of laser wavelength. Data have been obtained for (C3H6)n+ with n
in the range 2&#8211;15, and for (C5H10)n+ with n equal to 2 and 5. A comparison between the
cluster results and available infrared data on the various bulk-phase states of both
cyclopropane and 1,1-dimethylcyclopropane, suggests that the clusters are liquid-like.

J. J. Crooks

A. J. Stace

The Physical State of Clusters.
An Infra-red Laser Study of
Small Cyclopropane and
Dimethylcyclopropane Clusters

Single photon laser induced fluorescence (LIF) has been used to study the NO fragment
produced in its ground electronic state from the 355 nm photolysis of CH3ONO.
Populations of rotational, vibrational, spin-orbit and lambda doublet components have
been measured. The results are in broad agreement with previous two photon LIF
studies, and confirm the dynamics of the process in which the NO fragment departs from
an essentially planar CONO framework with non-statistical energy partitioning in the
internal states of the products.

C. G. Atkins

G.  Hancock

The 355 <small.letters>nm</small.letters> Photolysis of Methyl Nitrite

Reactive scattering of O atoms with Br2 molecules has been studied at an initial
translational energy E&#126;35 kJ mol&#8722;1 using cross-correlation time-of-flight analysis with
resolution improved over previous measurements. The centre-of-mass differential cross
section peaks in the forward and backward directions with a higher product translational
energy for backward Scattering. The angular distribution traced at the peak of the
product velocity distribution peaks more sharply in the forward than the backward
direction but the angular distribution of product flux shows a distribution which is more
nearly symmetrical about &#952; &#61; 90&#176;. The observed scattering is attributed to a triplet
OBrBr complex intermediate with a lifetime which is shorter than the period of overall
rotation of the axis of the heavy BrBr diatomic but which is long compared with the
period of vibrational and rotational motion of the light O atom.

N. C. Firth

N. W. Keane

D. J. Smith

R.  Grice

Reactive Scattering of Oxygen
Atoms With Bromine Molecules

The photodissociation of gaseous CClF2NO at 300 K and &#60;20 K in a supersonic jet has
been studied in the wavelength range 568 to 705 nm of the S1(n,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mi>&#x03C0;</mml:mi><mml:mo>*</mml:mo></mml:msup></mml:mrow></mml:math>)<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&#x2190;</mml:mo><mml:msub><mml:mi>S</mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math> electronic
transition. Energy disposal into, rates of formation and product-state distributions of the
nascent NO(X2<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mtext>&#x03A0;</mml:mtext><mml:mi>i</mml:mi></mml:msub></mml:mrow></mml:math>) photofragment have been measured in detail, and have been
compared with statistical (Prior) calculations. The rotational population distributions of
the NO fragment are statistical up to the highest excess energies investigated, while the
spin-orbit branching ratios are very non-statistical and non-thermal, favouring the lower
component. The lack of evidence for state-specific effects in the energy disposal and
appearance times of the NO indicate that excess energy randomises prior to dissociation.
Intersystem crossing to T1 dominates the predissociation of all but the lowest S1 levels.
Levels close to the O,O origin predissociate either by internal conversion to the ground
state or by vibrational predissociation on an S1 potential energy surface that is weakly
bound in the dissociation coordinate. A bond dissociation energy of 13 700 &#177; 350 cm&#8722;1
(164 kJ mol&#8722;1) has been evaluated for the C&#8212;N bond.

Martin R. S. McCoustra

Julie A. Dyet

Josef  Pfab

The State-selected
Predissociation Dynamics of
CClF2NO &#xC3;(n,<svg style="vertical-align:-0.1638pt" height="13.95" version="1.1" viewBox="0 0 17.775 13.95" width="17.775" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,13.688)"><path id="x1D70B" d="M574 449q-23 -58 -38 -79q-16 -4 -79 -4q-27 -100 -46 -219q-14 -87 7 -87t74 36l13 -27q-74 -81 -139 -81q-48 0 -48 62q0 22 8 59l60 258l-154 4q-21 -103 -54.5 -209t-64.5 -151q-38 -23 -83 -23l-7 15q46 36 94 152.5t64 216.5q-81 0 -138 -54l-18 23q22 27 39.5 43&#xA;t61.5 33.5t100 17.5q43 0 131.5 -2.5t129.5 -2.5q23 0 33.5 5t24.5 25z" /></g><g transform="matrix(.012,-0,0,-.012,10.113,5.525)"><path id="x2217" d="M471 153q-22 -15 -61 -13q-25 31 -45.5 49.5t-56.5 41.5q4 -71 28 -134q-17 -33 -42 -46q-24 12 -42 46q24 63 28 134q-36 -23 -56.5 -41.5t-45.5 -49.5q-36 -2 -61 13q0 28 19 59q65 10 130 43q-65 33 -130 43q-19 31 -19 59q22 15 61 13q25 -31 45.5 -49.5t56.5 -41.5&#xA;q-4 71 -28 134q17 33 42 46q24 -12 42 -46q-24 -63 -28 -134q36 23 56.5 41.5t45.5 49.5q36 2 61 -13q0 -28 -19 -59q-65 -10 -130 -43q65 -33 130 -43q19 -31 19 -59z" /></g></svg>)

Sustained visible emission in the region 440&#8211;850 nm from the B <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&#x2192;</mml:mo></mml:math> X system of IF is
observed when a gas phase mixture (&#126;0.5 mbar) of an alkyl iodide with F2 in He is
photolysed at 248 nm by a KrF laser. The total intensity and decay rate of the IF(B)
emission is a strong function of the identity of the alkyl iodide and can be correlated with
the 248 nm photon yields for the production of I*(2p1/2). The half-lives for the IF(B)
decays range from 5 &#956;s for t-C4H9I to 770 &#956;s for n-C3F7I. Decay curves for the I*(2p1/2)
concentrations are also measured by atomic fluorescence. The mechanism for IF(B)
formation in these systems is discussed and it is suggested that IF(B) is produced either
by a recombination process involving I* and F atoms or by multistep collisional
excitation of ground state IF(X) by I*. The F atoms can be produced following the
photolysis pulse by the reaction of the alkyl radical with molecular fluorine and IF(X)
can result either from the reaction of F atoms with the alkyl iodide or from a dark
reaction between molecular fluorine and the alkyl iodide.

D.  Raybone

 T. M. Watkinson

J. C. Whitehead

F.  Winterbottom

The Production of IF(B3
<svg style="vertical-align:-0.0pt" height="11.175" version="1.1" viewBox="0 0 14.6875 11.175" width="14.6875" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg"><g transform="matrix(.017,-0,0,-.017,.062,11.113)"><path id="x1D6F1" d="M840 650l-8 -28q-60 -5 -77 -20.5t-28 -75.5l-76 -402q-11 -60 3 -76t81 -20l-5 -28h-280l6 28q63 5 81.5 20.5t29.5 75.5l88 461q3 17 -3 22t-27 5h-288q-20 0 -26 -5t-10 -25l-87 -458q-11 -60 3 -76t85 -20l-6 -28h-279l5 28q61 4 79 20t29 76l76 402q11 60 -2 75.5&#xA;t-83 20.5l6 28h713z" /></g></svg>) in the
248 nm Laser Photolysis of
Fluorine/Alkyl Iodide Mixtures

The methyl iodide A-band photodissociation is investigated using a photofragment
time-of-flight technique coupled with state specific detection of either the iodine atom or
methyl radical dissociation products by multiphoton ionization. The method is quite
general and allows the fragments&#39; velocity and spatial distributions to be determined
simultaneously with their rotational, vibrational and electronic excitation. It is shown
that methyl product from two competing dissociation channels, one direct, the other
necessitating an initial surface crossing, can be clearly distinguished by virtue of the
characteristic translational excitation in each. The different rotational excitation which
is observed in these two channels is discussed in terms of the dissociation dynamics in the
methyl iodide A-band continuum.

John F. Black

Ivan  Powis

A Simultaneous Photofragment Spectroscopy Measurement of Rovibronic and Translational Excitation in the CH3l A&#8211;Band Photodissociation System

Two-photon spectra of the S1(&#195;)&#8211;S0(<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mover accent="true"><mml:mi>X</mml:mi><mml:mo>&#x02DC;</mml:mo></mml:mover></mml:math>) transition of jet-cooled ortho-, meta- and
para-xylene have been recorded in the region 43,000&#8211;37,000 cm&#8722;1 using resonance
enhanced multiphoton ionization excitation. A full vibronic analysis for all three isomers
has been carried out enabling the identification of several excited state vibrational
frequencies for the first time. In particular, for ortho and para substitutions, a new,
strong two-photon vibronic coupling mode <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>&#x03BD;</mml:mi></mml:math>19b has been identified, and the significance
of this result is discussed. Some anomalies in the assignment of previously published
spectra of the xylenes, and related disubstituted benzenes, have been observed and
reassignments are suggested. In addition we show that the comparison of one and
two-photon spectra provides a method for determining the mechanism by which totally
symmetric fundamentals gain their intensity in one-photon spectra, namely either from
Herzberg&#8211;Teller intensity stealing or due to favourable Franck&#8211;Condon overlap.

T. G. Blease

R. J. Donovan

P. R. R. Langridge-Smith

T.  Ridley

(2 + 2) Resonance Enhanced
Multiphoton Ionization Spectra of
Jet-Cooled <small.letters>ortho</small.letters>-, <small.letters>meta</small.letters>- and
<small.letters>para</small.letters>-Xylene

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Laser Chemistry in the UK

Published Special Issues

Laser Chemistry

Laser Chemistry in the UK

Laser Chemistry in the UK

Articles

Surface-Enhanced Raman Spectroscopy of Thionine-modified Gold Electrodes

Laser Induced Fluorescence of 4-N,N-Dimethylaminobenzonitrile (DMABN) and Related Compounds

Velocity Dependence of Rotationally Inelastic Cross-Sections

The Physical State of Clusters. An Infra-red Laser Study of Small Cyclopropane and Dimethylcyclopropane Clusters

The 355 nm Photolysis of Methyl Nitrite

Reactive Scattering of Oxygen Atoms With Bromine Molecules

The State-selected Predissociation Dynamics of CClF₂NO Ã(n,)

The Production of IF(B³ ) in the 248 nm Laser Photolysis of Fluorine/Alkyl Iodide Mixtures

A Simultaneous Photofragment Spectroscopy Measurement of Rovibronic and Translational Excitation in the CH₃l A–Band Photodissociation System

(2 + 2) Resonance Enhanced Multiphoton Ionization Spectra of Jet-Cooled ortho-, meta- and para-Xylene

Laser Chemistry

Laser Chemistry in the UK

Laser Chemistry in the UK

Articles

Surface-Enhanced Raman Spectroscopy of Thionine-modified Gold Electrodes

Laser Induced Fluorescence of 4-N,N-Dimethylaminobenzonitrile (DMABN) and Related Compounds

Velocity Dependence of Rotationally Inelastic Cross-Sections

The Physical State of Clusters. An Infra-red Laser Study of Small Cyclopropane and Dimethylcyclopropane Clusters

The 355 nm Photolysis of Methyl Nitrite

Reactive Scattering of Oxygen Atoms With Bromine Molecules

The State-selected Predissociation Dynamics of CClF2NO Ã(n,)

The Production of IF(B3 ) in the 248 nm Laser Photolysis of Fluorine/Alkyl Iodide Mixtures

A Simultaneous Photofragment Spectroscopy Measurement of Rovibronic and Translational Excitation in the CH3l A–Band Photodissociation System

(2 + 2) Resonance Enhanced Multiphoton Ionization Spectra of Jet-Cooled ortho-, meta- and para-Xylene

The State-selected Predissociation Dynamics of CClF₂NO Ã(n,)

The Production of IF(B³ ) in the 248 nm Laser Photolysis of Fluorine/Alkyl Iodide Mixtures

A Simultaneous Photofragment Spectroscopy Measurement of Rovibronic and Translational Excitation in the CH₃l A–Band Photodissociation System