Optical cooling delay

The solution was contained in a cylindrical Pyrex or silica optical cell, 2.5 cm. in diameter and 2 cm. optical depth, fitted with a side arm of diameter about 1 cm. for connection to the vacuum line (see Fig. 10). Both phosphorescence and delayed fluorescence in fluid solution at room temperature are quenched by exceedingly low concentrations of molecular oxygen, and efficient deaeration is of the utmost importance. The following procedure was found to be satisfactory. The cell was first cooled in an acetone/C02 bath and pumped out. It was then isolated from the pump, refluxed to remove dissolved gas, cooled again in the... 

An ultrafast time-resolved near- and mid-IR absorption spectrometer was designed to achieve high sensitivity, ultrafast time resolution, and broad tunability in the near- and mid-IR regions (see Fig. 2). The details of this spectrometer are described elsewhere (9). Briefly, MbCO was photolyzed with a linearly polarized laser pulse, whose polarization direction was controlled electronically by a liquid crystal polarization rotator. The photolyzed sample was probed with an optically delayed, linearly polarized IR pulse whose transmitted intensity was spectrally resolved with a monochromator and detected with either a Si photodiode (near-IR RilO cm-1 bandpass) or a liquid nitrogen-cooled InSb photodetector (mid-IR 3 cm-1 bandpass). To measure the sample transmission, this signal was divided by a corresponding signal from a reference IR pulse... 

Time-gated detectors that allow the optical emission from the laser plasma to be recorded at some time delay after the laser pulse are required to accurately capture the emission spectra. For the first few microseconds after the ignition of the laser spark, the plasma emits a strong white light continuum (also called bremsstrahlung), which decays as the plasma cools the characteristic atomic and ionic emission lines only appear as the plasma cools. A detector delay on the order of several microseconds after the laser pulse is used to eliminate interference from the continuum radiation. The principle is demonstrated in Figure 7.52. 

To obtain a high accuracy the detector channel matrix is cooled by a Peltier element to get a signal-to-noise ratio more than 100. Moreover, we average 40 test and reference spectra to determine the difference spectrum at each delay setting. From the transmission measurements with excitation (we) and with no excitation (ne) the difference of the optical density AD(l,f) is calculated by ... 

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