Chapter 2. Experimental
2.4 SYSTEM OPERATION
To do an ion spectroscopy experiment, we carry out the following procedures.
First, the operations of the ion apparatus and the OPO are optimized independently.
Then, both the pump laser of the OPO and the Stanford Research System digital delay generator (model DG-535), which controls the pulse sequence of the ion apparatus, are switched to external triggering by an EG&G digital delay generator (model 9650).
Finally, the delay time between the pulse sequences of the ion apparatus and the OPO is adjusted so that the selected ions and the OPO beam arrive at the laser interaction spot simultaneously. Only then are we ready for an infrared spectroscopic scan of the selected ion.
2.4.1 Ion Apparatus Operation
The pulse sequence of the ion apparatus is controlled by the Stanford Research System digital delay generator. A TTL-pulse from delay channel A triggers the pulsed valve driver. If the pulsed discharge ionization is used for the experiment, channel B goes to TTL-high -200 ps later, triggering the thyratron, which initiates a discharge across the Ni electrodes. If the electron beam ionization is used, channel B is not used
C goes to TIL-high, triggering the TOF pulsed repeller circuit, marking the starting-time for the flight of the ions in the mass spectrometer. (Channel C is also connected to the trigger input of the mass gate controller, which has an internal delay circuit.) The resulting TOF mass spectrum is collected by the LeCroy waveform digitizer, which starts its data acquisition upon reception of a triggering pulse from channel D (usually set at 20 ps after the channel C pulse).
A TOF waveform trace is collected by the LeCroy with a single pulsing event.
But to obtain a mass spectrum with a good signal-to-noise ratio, we need to collect the data for a large number of pulsing events and signal-average the data. This is accomplished by using an IBM-PC computer (nicknamed "TOFU"), which runs a computer program named TOFVJEW2. To collect data, "TOFU" activates the LeCroy waveform digitizer and sets its accumulative event counter Ns through a GPIB interface bus. (Usually Ns is set to 100, as a much larger number may cause data overflow in the memory buffer). The LeCroy accumulates the digitized TOF waveform trace into its memory buffer until its scan counter reaches zero, after which it stops and waits for the data transfer command from the "TOFU" computer. Upon the reception of that command, the TOF waveform data are retrieved to the computer memory and subsequently displayed on the monitor by TOFVIEW2. The LeCroy is then reactivated for the next Ns events.
The preceding procedure repeats N/Ns times until the total event number N is reached.
The resulting final TOF mass spectrum can be saved, displayed, and printed out using TOFVIEW2. The intensity of each individual mass peak can also be calculated (integrated) automatically by TOFVIEW2.
33 2.4.2 The Pulse Sequence of the OPO
The Nd:YAG laser which pumps the OPO can be triggered by its internal pulse circuit, or externally by the EG&G delay generator. When the laser is externally triggered, channel A of the EG&G triggers the flashlamp, and channel B triggers the Q- switch firing of the laser. Typical delay-times involved are: A - T0
=
1000 ps, B -A=
120-160 ps. A laser beam at 1.06 pm emerges from the cavity of theY AG laser a few nanoseconds after the Q-switch firing; and after an additional delay of -5 ns, the OPO is lasing.2.4.3 Synchronization Between the Ion Apparatus and the OPO
Unless both the parent ion beam and the OPO beam arrive at the laser interaction spot simultaneously, vibrational predissociation will never occur. The synchronization of these two beams is achieved by using a master delay generator for the whole experiment, and finding the correct delay-time.
In our full experiment, the EG&G is the master delay generator. Besides triggering the pump laser of the OPO, it can also control the pulsing of the ion apparatus by connecting the output of its channel T0 to the external-trigger input of the Stanford Research System delay generator. The SRS thus becomes a slave delay generator which is externally triggered by the EG&G master, and the adjustable delay-time which is responsible for the beam synchronization is the delay between channel A of the SRS and the SRS external-trigger input.
To get the correct delay-time at channel A that leads to beam synchronization, we carry out the following procedure. We first position a MCP ion detector, which is mounted on a motion feedthrough, at the laser interaction spot. With an oscilloscope, we
with a photodiode. The correct delay-time is found by adjusting the delay so that these two signals appear at the same time on the oscilloscope screen.
2.4.4 The Complete Experiment
The flowchart for the whole experiment is shown in figure 14. The ion infrared spectrum is recorded by stepping the wavenumber of the OPO beam and averaging the photofragment intensity for 400 pulsing events at each OPO wavenumber. Background fragment ion intensity is subtracted by firing the laser beam alternately at the parent ion arrival time (IN SYNC) and 10 ps before the ion arrival (OUT OF SYNC), and taking the difference. The fragment intensity data are sent from "TOFU", through a serial port connection, to the "LASER" computer where they are normalized with respect to the OPO laser intensity.
The procedure described above generates a vibrational predissociation spectrum of the parent ions. Usually, the signal-to-noise ratio of one spectrum is not satisfactory.
To achieve a better signal-to-noise ratio, we repeat the experiment for about 10 times and average the resulting spectra.
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