Hardware Summary

Laser System

• Pulsed Nitrogen Laser (337.1 nm)

  • PTI Model 2300
    • This is several times more powerful and has shorter pulse duration than several other nitrogen lasers on the market (Oriel, LSI.) The PTI model is physically much larger, it requires an external nitrogen line, and it operates with a spark gap rather than a thyratron. (Spark gaps generate high levels of RF in the 100-500 MHz region; this requires careful attention to shielding on cables and electronic circuits.)
  • 1.4 millioule/pulse (2.3 MWatt)
  • 600 nsec pulses
  • 337.1 nm, 0.04 nm bandwidth
  • 10-20 Hz repetition rate
  • beam is 3x6 mm
  • Overall dimensions: 30 x 20 x 9 inches

    ---

  

• Dye Laser

  • PTI Model PL202
  • Tunable (360-900 nm with suitable laser dye) ; manual tuning
  • band width nominally 0.04 nm
  • typically 100-150 microjoules/pulse (depends on dye used)
  • overall dimensions: 21 x 20 x9 "
  • The PL202 is a high resolution dye laser. It contains two dye cells.
    • The first serves as an oscillator. It is located in an optical cavity formed by a mirror and an adjustable grating in a grazing incidence cavity. It receives 40% of the light from the nitrogen laser.
    • The second dye cell serves as an amplifier; it receives 60% of the light from the nitrogen pump laser.

• Monochromator

  
  • PTI model 01-001
  • This is a 0.25 meter Monochromator, f4
  • Configuration: Czerny-Turner
  • dual grating, 1200 lines/mm (blazed at 360 and 500 nm)
  • Minimal usable bandwith: 0.25 nm (4 nm/ mm slit width)
  • Equipped with stepping motor drive, computer controlled

    ---

• Optical Detection-- Photomultipier

  • Hammasatu 928 ($400 approx.)
    • 1.1 " diameter, side window
    • Multialkali type: useful optical range 185-900 nm.
    • rise time-- 2.2 nsec (determines pulse resolution)
    • transit time -- approximately 22 nsec
  • Power supply
    • C4900 Hamasatu dc to dc converter
      • 15 v supply
      • output voltage controlled by resistance or dc level
      • 200-1500 V output
    • Conventional xxxx High Voltage Supply
  • PIN-Si Diode (for pulse detection circuitry)
    • We use very inexpensive devices (Panasonic PN-334PA, 2 nsec rise time, about $1.40, DigiKey Electronics).
    • These have very limited UV response, peaking at 900 nm, but there is generally enough stray UV/visible signal to use.
    • An alternative is to let PIN diode view a fast fluorescence screen exposed to the beam.
  • Diode Array Spectrometer
    • Ocean Optics (fiber optics spectrometer)
      • Light is collected via 50 micron quartz fiber
      • Light is dispersed by a grating
      • Light is detected by a 1000-2000 element CCD Array
      • Entire units fits on a half sized PC card
      • Data collection, display is controlled by the PC

      ---

• Signal Handling (fast transients)

  • Tektronix TDS380 Digital Oscilloscope
    • The oscilloscope, with the communications interface costs about $4000-4500
    • 0.5 nsec max acquisition rate; 400 MHz
    • 1000 data points/channel; two channels
    • complete computer control and data access via RS232 port, GPIB or Centronix Parallel Port.
      • We generally sychronize data collection using a PIN photodiode exposed indirectly to the nitrogen laser pulse. This goes into channel 2 of the oscilloscope and it can be monitored as a measure of pulse amplitude.
      • A PMT provides the usual signal on channel 1.
      • For very fast processes (like Raman Spectra) we often select and integrate only the early portion of the PMT signal. This is comparable to using a Gated Integrator (Box Car Integrator.) This provides improved S/N ratios by rejecting most fluorescence.
  • PMT Amplifiers
    • locally constructed with xxxx Op Amp
      • Nanosecond signals make serious demands on amplifiers; a minimum bandwidth of 250 MHz is required. Most operational amplifiers (and most circuit designs) do not function in this frequency domain
    • We have generally required double shielded coax cables. We use standard coax and encase the line in a woven copper shield. (This reduces stray RF by at least a factor of 10.)

      ---

• Software (locally developed)
  • tutorial software (simulations, hardware concepts)
  • control software
    • Monochromator drive software
      • The monochromator drive motor operates from a PC card. The control card is memory mapped and the program uses a series of PEEK and POKE statements (in Quick Basic) to control the device.
      • Visual Basic does not support PEEK and POKE commands; we use a shareware DLL package that provides equivalent calls.
      • The drive card generally will accept commands to drive to a specific wavelength; it is not necessary for the application program to issue discrete stepper motor pulses.
    • Oscilloscope control
      • Oscilloscope control and data transfer occurs via the RS232 Serial Port.
      • Faster processing is possible using the GPIB or Centronix (parallel port) interfaces; since we are limited by the laser repetition rate the RS232 port is satisfactory.
      • Tektronix provides sample programs for oscilloscope control; we have modified these for our specific use.
    • Laser Control
      • The nitrogen laser can be fired with a TTL pulse. (We generate this with a spare line within the monochromator drive.
  • data acquistion and control
    • Raman spectra (pseudo gated integrator)
      • It is possible to record Raman Spectra for neat (100%) liquids. At short wavelengths the Raman intensity is much higher, so the 337 Nitrogen laser can generate relatively strong signals. One serious drawback is fluorescence which can seriously mask the Raman. The program below uses time resolution to reject most of the longer lived fluorescence signal.
        • Software begins by setting the monochromator to the starting wavelength (typically 340 nm.)
        • The software then fires the laser once.
        • The oscilloscope triggers (PIN diode sensor) and it records the PMT signal. At the fastest rate (0.5 nsec/point) the trace covers a 0-500 nsec period.
        • The oscilloscope is operating in Signal Averaging mode so it will automatically average multiple laser pulses. The previous two steps are repeated 16-64 times.
        • After sampling N-flashes, the oscilloscope signal is transferred to the computer.
        • Only the initial portion of the signal is kept. Because the PMT signal is delayed about 20 nsec we typically only accept data from 15-25 nsec. This portion of the signal is integrated to provide one point in the spectrum.
        • The monochromator is advanced (typically 1-2 nm) and the cycle is repeated until the desired spectral region has been recorded.
    • Spectra
    • Fluorescence Spectra and Lifetime measurements
  • Ocean optics Spectrometer
    • We have used an Ocean Optics S2000 Spectrometer to record emission spectra using 337 nm excitation. Relatively long integration time (1000-2000 msec) are used with the laser operating around 10 Hz. Thus we integrate over 10-20 laser pulses. The optical fiber is simply placed 90This is a simple way to record strong emission spectra such as fluorescence. Of course there is scattered light at 337 nm (and a secondary signal appearing at 674 nm.)
    • This is much faster than using the monochromator and PMT, stepping through a range of monochromator settings.
    • We are trying to use the S2000 to record an absorption spectrum of a transient species.
      • This is really an electronic analog of the Porter flash spectroscopy experiments of the 1950's.
      • The Nitrogen laser pulse intiates the reaction.
      • A Xenon flash lamp serves as the absorption beam.
      • The diode array spectrometer records the transmitted signal.
        • An electronic delay circuit controls when the Xenon lamp fires and thus determines when the absorption spectrum is measured.
      • Multiple flashes are accumulated to improve the signal intensity.
      • The spectrum can be recorded with different delay times to get information on the lifetime of the transient species.
        • A practical limit is the duration of the Xe flash. This is approximately 1 microsecond so it limits studies to species with microsecond lifetimes.
    • Languages
      • Initial software is in Quick Basic (DOS)
      • Upgraded Software in Visual Basic
      • (attempts at virtual lab software.. National)

  ---

  return to project

  return to home page