Laser Spectroscopy and
Nanoparticle Research
at The University of Texas in Austin
Nanoparticle Research
at The University of Texas in Austin
Laser Spectroscopy in Supersonic Jets
The invention of the LAM proces allows us to perform laserspectroscopy on cold semiconductor cluster beams. We can manufacture clusters in supersonic jets by flowing a powder aerosol into a flow reactor. Clusters were then formed by laser ablation of the microspheres in the gas flow prior to nozzle expansion. A schematic of the apparatus is shown in Fig. 1. The aerosol generator suspends microparticles in a stream of inert gas in a flow reactor. The gas can be precooled to temperatures near 130K. An excimer laser will then be used to produce semiconductor nanocrystals using the LAM process. Because only a small fraction of the laser light is absorbed, the temperature increase of the gas is less than 20K and the hot nanoparticles will cool by gas collisions during travel to the supersonic nozzle.
Expansion through the nozzle will further cool the clusters, and the jet will be skimmed to form a supersonic beam. We will then use tunable lasers to measure both resonance enhanced multiphotoionization (1+1REMPI) and fluorescence of the clusters. Fluorescence spectra of selectively excited resonances, determined by REMPI, will also be measured. Approximate information about the size of the clusters excited can be determined by measuring the mass of the photoion by time-of-flight (TOF).
Fig.1 Supersonic Beam Machine
skimmer isolates this stagnation chamber from the second, laser interaction chamber. The third chamber - detection chamber- houses a quadrupole mass spectrometer (Extranuclear, Inc.) with a mass range of 400 amu and resolution of 0.5 amu. A perpendicular extension of the second chamber houses a TOF spectrometer. The vacuum system has been designed for ultra high vacuum in order to minimize ionization of the background gas when using UV lasers and resonance ionization detection. The laser is focused diagonally into the laser interaction chamber through a baffled window and is dumped into a high-quality Wood's horn to minimize scattered laser light. Fluorescence is collected in both the forward and backward directions and focused onto an imaging spectrometer employing a liquid N2 cooled, back-etched CCD camera for a detector. Because the detector cannot see the walls of the chamber, laser scattered light is further reduced. A mask placed at the first image plane is used to reduce the viewing area and further reduce the scattered laser light. The detector has 60-85% quantum efficiency in the 600-850 nm range (total range 250 to 1000nm), a dark noise of a negligible 0.1 e-/hour, and a readout noise fluctuation of only 1.5 e-/pixel. Because the detector is two dimensional, we are able to image the excited jet. Specific regions of the detector can then be selected by software; which enables us to improve the rejection of scattered laser light as the excimers fluoresce downstream from the laser interaction region. Experimental tests have measured less than 3 photons of scattered laser light per 1000 laser shots.
The ion detector is a deflection, double-field, time-of-flight mass spectrometer for use with ions produced by REMPI. This technique is very sensitive and enables the measurement of excitation spectra of excited molecules and clusters unstable for fluorescence.
Two types of tunable lasers are available for these experiments in our laboratory. The laser used for most of our spectroscopy is a Hänsch type laser. The basic oscillator has a computer controlled, echelon grating and provides 10 GHz resolution. For spectral scans with 400 MHz resolution, a zerodur-spaced etalon is inserted. The etalon is rapidly pressure scanned using a computer controlled piston. By synchronizing the etalon and computer, we make spectral scans over a large range. By having two stages of resolution, survey scans can be made using a broad band oscillator with high resolution spectra obtained in regions of interest. This laser can be frequency doubled using a BBO crystal, providing up to 10 mJ of light from 300 to 220 nm.
A second tunable laser system uses pulsed amplification of the output of a single mode, Ti-sapphire oscillator (Coherent 899-21). This laser output will
be frequency tripled to obtain tunable UV light. Higher energy transitions can be excited in two-photon transitions. The laser can operate over the wavelengths from 670 to 1000 nm; the output will be doubled in two KDP crystals and then tripled in BBO crystals to provide light from 335-500nm in doubled mode (estimated energy output of 4-8 mJ ) and from 223-333nm in tripled mode (estimated energy output of 1-2 mJ). By controlling the number of passes in the amplifier rods, the pulse-width of the laser can be controlled up to a FWHM of 20 ns. By maintaining a Fourier-transform limit for the amplifier output, we estimate a resolution of 20 MHz.
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