1,4,8,11,15,18,22,25-Octaoctylphthalocyanine shows the highest time-of-flight (“long-range”) hole mobility so far reported for the columnar phase of a discotic liquid crystal. Unlike most other high long-range mobility columnar discotics, there is no clear evidence from x-ray diffraction of high order.

We consider the highly radiative, long-lived photoluminescence (PL) component observed in colloidal CuInS 2 /ZnS core/shell quantum dots (CIS/ZnS QDs) and provide evidence of the involvement of intra-gap defect states in the emission, settling a long ongoing discussion in the literature. Femtosecond transient absorption (fs-TA) spectroscopy was used to investigate sub-picosecond dynamics in these technologically important QDs. Spectral and kinetic analysis of the fs-TA data, in combination with femtosecond pump-dump-probe (PDP) experiments, revealed a stimulated emission (SE) component in CIS/ZnS QDs for the first time. PDP experiments showed that the excited state absorption (ESA) signal, originating from the conduction band (CB), was immune to the depopulation of 2 the emitting state by a third, 'dump' laser centered close to the luminescence maximum. We conclude that the optical transition responsible for the observed room-temperature PL in CIS/ZnS QDs cannot originate from the CB as postulated in the literature, but rather from high-lying intraband donor states most likely associated with indium-copper anti-site defects. Filling of the emitting sub-bandgap state was assigned with a time constant of 0.5 ps and de-excitation via remaining surface states was associated with a 1.8 ps time constant. A third longer decay constant (27 ps) was attributed to Auger recombination.3

Rotational wave packet revivals on an excited electronic state have been measured by femtosecond time-resolved photoelectron imaging for the first time. The first full revival at 82 ps of S1 (n,pi*) pyrazine was clearly observed in the time dependencies of the photoelectron intensity and the photoelectron angular distribution (PAD). The PAD, measured for laser aligned pyrazine, clearly reflects the different characters of pi* and 3s molecular orbitals.

We have studied dissociative multiphoton ionization of NO2 by time-resolved velocity map imaging in a two-color pump-probe experiment using the 400 and 266 nm harmonics of a regeneratively amplified titanium-sapphire laser. We observe that most of the ion signal appears as NO+ with approximately 0.28 eV peak kinetic energy. Approximately 600 fs period oscillations indicative of wave packet motion are also observed in the NO+ decay. We attribute the signal to two competitive mechanisms. The first involving three-photon 400 nm absorption followed by dissociative ionization of the pumped state by a subsequent 266 nm photon. The second involving one-photon 400 nm absorption to the 2B2 state of NO2 followed by two-photon dissociative ionization at 266 nm. This interpretation is derived from the observation that the total NO+ ion signal exhibits biexponential decay, 0.72 exp(-t/90+/-10)+0.28 exp(-t/4000+/-400), where t is the 266 nm delay in femtoseconds. The fast decay of the majority of the NO+ signal suggests a direct dissociation via the bending mode of the pumped state. .

An instrument to detect atmospheric concentrations of the hydroxyl (OH) and hydroperoxyl radicals has been developed (HO 2 ) using the FAGE (Ñuorescence assay by gas expansion) technique. The instrument is housed in a mobile laboratory and monitors the OH radical via on-resonance laser-induced Ñuorescence (LIF) spectroscopy of the A 2&`(vA \ 0) transition at (v@ \ 0)ÈX 2% i ca. 308 nm. Ambient air is expanded through a 1 mm nozzle to low pressure where it is irradiated by the laser pulse at a repetition rate of 7 kHz, with the resultant Ñuorescence being detected by gated photon counting.is monitored by chemical HO 2 conversion to OH by the addition of NO, with subsequent detection using LIF. Following laboratory and Ðeld calibrations to characterise the instrument sensitivity, detection limits of 1.8 ] 106 and 2.1 ] 107 molecule cm~3 were determined for OH and respectively, for a signal-to-noise ratio, S/N, of 1 with 150 s signal integration time. The instrument was deployed for the HO 2 Ðrst time during the ACSOE Ðeld campaign at Mace Head, Eire, for which illustrative results are given.

Time-resolved photoion and photoelectron velocity mapped images from NO(2) excited close to its first dissociation limit [to NO(X(2)Pi) + O((3)P(2))] have been recorded in a two colour pump-probe experiment, using the frequency-doubled and frequency-tripled output of a regeneratively amplified titanium-sapphire laser. At least three processes are responsible for the observed transient signals; a negative pump-probe signal (corresponding to a 266 nm pump), a very short-lived transient close to the cross-correlation of the pump and probe pulses but on the 400 nm pump side, and a longer-lived positive pump-probe signal that exhibits a signature of wavepacket motion (oscillations). These transients have two main origins; multiphoton excitation of the Rydberg states of NO(2) by both 266 and 400 nm light, and electronic relaxation in the 1(2)B(2) state of NO(2), which leads to a quasi-dissociated NO(2) high in the 1(2)A(1) electronic ground state and just below the dissociation threshold. The wavepacket motion that we observe is ascribed to states exhibiting free rotation of the O atom about the NO moiety. These states, which are common for loosely bound systems such as a van der Waals complex but unusual for a chemically-bound molecule, have previously been observed in the frequency domain by optical double resonance spectroscopy but never before in the time domain.

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We review the literature concerning the electronic structure and spectroscopy of nitrogen dioxide for excitation energies up to 20 eV. Our aim is not to be exhaustive but rather to summarize important results and observations which we have found useful in the interpretation of recent experiments in both the frequency and time domain in which competing and often multiphoton excitation paths access high lying Rydberg and valence states. The photodynamics in NO 2 are particularly fascinating and complicated by numerous non-adiabatic couplings between the various electronic states which, despite the molecule's apparent simplicity as a triatomic species, leads to a very rich photochemistry that exemplifies many features occurring in the photochemistry of much larger molecules.