Quantum interference in NO2.

This paper investigates the origin of a quantum interference observed when NO(2) is dissociatively ionized by short pulses of ultraviolet light. We describe time-resolved measurements of NO(+), O(+), and NO(2)(+) ions produced following the interaction of NO(2) with a approximately 70 fs duration pulse centered close to 400 nm and a subsequent time-delayed probe pulse close to 269, 205, or 400 nm. A quantum beat oscillation with a period of 524 fs and a characteristic damping time of 8 ps is observed on all transient ion signals. We investigate the effect of tuning the central wavelength of the excitation pulse over a 12 nm range, and we discuss the potential importance of three possible multiphoton pathways involving one, two, and three pump photons. We conclude that the ionization pathway responsible for the beat signal is most likely due to a process involving the absorption of two pump photons and two probe photons. This presents an interesting problem with respect to the interpretation of the mechanism responsible for the quantum interference signature since the electronic states of NO(2) reached at the two-photon level are all thought to be extremely short-lived and to dissociate on a time scale that is far shorter than the characteristic damping time of the oscillatory signals. We suggest that a possible explanation for the observed dynamics is associated with a minor dissociation channel of the (2)(2)B(2) state of NO(2) through its interaction with the longer lived (2)(2)A(1) state.

Photoelectron imaging on time-dependent molecular alignment created by a femtosecond laser pulse.

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.

Temperature fields during the development of autoignition in a rapid compression machine.

Temperature and concentration fields have been investigated in the cylindrical combustion chamber of a rapid compression machine (RCM) by schlieren photography, chemiluminescent imaging and planar laser induced fluorescence of acetone and of formaldehyde in a 2-dimensional sheet across the diameter. The timescale of particular interest was up to 10 ms after the piston has stopped. Experiments were performed in non-reactive and reactive conditions. Acetone was seeded in non-reactive mixtures. Combustion was studied first in a system containing di-tert-butyl peroxide vapour in the presence of oxygen. The decomposition of di-tert-butyl peroxide generates methyl radicals, which are then oxidised if oxygen is present. The overall reaction is exothermic and is characteristic of a conventional thermal ignition. In addition, chemiluminescence, resulting from CH2O*, accompanies the oxidation process. The combustion of n-pentane was then investigated at compressed gas temperatures that spanned the range in which there is a negative temperature dependence of the overall reaction rate, typically 750-850 K. The response to thermal feedback in this more complex thermokinetic system can be the opposite of the "thermal runaway" that accompanies di-tert-butyl peroxide combustion. The purpose of making comparisons between these two types of systems was to show how the temperature field generated in the RCM is modified in different ways by the interaction with the chemistry and to discuss the implications of this for the spatial development of spontaneous ignition. As the piston of the RCM moves it shears gas off the walls of the chamber. This probably creates a roll-up vortex, but more importantly it also collects gas from the walls and moves it across the cylinder head pushing it forward into a plug at the centre. Thus, soon after the end of compression there is an adiabatically heated gas which extends virtually to the wall, but this incorporates a plug of colder gas at its core. Diffusive transport will occur, but the timescale is relatively slow, and the effect hardly shows until at least 10 ms post-compression. The consequence of "thermal runaway" on a timescale that is compatible with the development of this temperature field is that the reaction rate in the adiabatically compressed toroidal region accelerates faster than in the core, and goes to completion first. A somewhat similar pattern emerges during n-pentane combustion when the initial condition is set at the lower end of the negative temperature dependent range. By contrast, at adiabatically compressed gas temperatures close to the upper end of the negative temperature dependent region, the reaction rate in the cooler core develops faster than that in the surrounding zone, and the temperature difference is rapidly smoothed out. This does not lead to spatial homogeneity in all respects, however, because different rates and extents of reaction generate different concentrations of intermediates. This stratification has implications for the eventual spatial evolution of spontaneous ignition.

Outbreak of tuberculosis in a church.

A 48-year-old white man (index case) with an abnormal chest radiograph remained undiagnosed for tuberculosis for 4 years. Investigation by purified protein derivative (PPD) tuberculin test revealed positive tuberculin reactions (> or = 10 mm induration) in seven of eight (88%) initial close contacts, and in 12 of 46 (26%) coworkers. On the suspicion that transmission had also occurred among members of the index case's church congregation, a PPD tuberculin test survey of 184 of 200 of the members revealed 77 (42%) positive reactors. Thirty percent of the members under the age of 35 years were infected, suggesting transmission of infection. Eight cases of active tuberculosis (including the index case) were detected, yielding a high case rate (4.3%) among the parishioners. Three of the cases were confirmed recent PPD converters. Although bacteriologic findings were available in only three of the eight cases, two cases had phage typing of organism identical to the index case; the third had recrudescent tuberculous disease. Of the remaining five cases without bacteriologic confirmation, two had pleural tuberculosis, one child had progressive primary tuberculosis, and two persons had localized pulmonary nodules suggestive of primary infection progressing to disease. Because transmission of tuberculous infection may occur in any closed environment, including a church, physicians must be conversant with tuberculosis control measures and preventive therapy guidelines to preclude unforeseen transmission of disease.