Timing is controlled in this experiment via the SRS photon counter, with one of the two pump Nd:YAG lasers serving as the master. Two possible triggering schemes are shown schematically in Figure 2-7. “Vic’s box” is the historical name for a piece of electronics hardware that is used to divide the frequency or switch the polarity of the signal directed through it, depending on the triggering scheme in use. The operation of Vic’s box is outlined in Figure 2-8.
The output of Gate A is used to control the pump-probe delay of the system. Gate B is optimized to reject the UV LIF probe pulse scatter, but to collect the maximum amount of OH LIF signal. The gate delays and widths (for A and B) can be adjusted in the SRS by touching “COM,” then selecting A or B, using the up and down arrows to select gate delay or width, and then using the black turning knob to change the value.
For the 100 Hz mode, the Infinity laser is the master trigger. The Infinity software must be set on triggering “internal,” and the SP YAG on trigger pulse repetition frequency
(PRF) = 0. This is its default setting for accepting an external trigger. The Infinity Q- switch sync is input to the SRS, and the Gate A output is channeled into Vic’s box. The Channel 1 output with up polarity (the right Channel 1 BNC port in Figure 2-8) is sent to the SP trigger in. No delays or frequency dividing are used on Vic’s box.
For the mode with 100 Hz pump and a 6400 Hz “comb” of probe pulses, the SP YAG is the master trigger. The Infinity software must be set on “external” (be careful not to trigger at too high a repetition rate! There is no guard against this!), and the SP YAG trigger PRF = 6400. The SP YAG sync out is directed into both Vic’s box and into the SRS. Vic’s Channel 2 out with down polarity (the left Channel 2 BNC in Figure 2-8), which generates the division to 100 Hz repetition rate, is sent to the Infinity lamp trigger in. Check this rep rate with the oscilloscope before triggering the Infinity off it. The SRS timing can be adjusted such that Gate A is zero and Gate B comes just after the probe laser pulse. Vic’s box delay is used to move Gate B so that it does not include the pump laser pulse.
The most common experiment done in this thesis was the 100 Hz experiment with the Infinity as the master trigger. For these experiments, good initial settings for the SRS are Gate A and Gate B delays separated by about 1.1 µs to reject UV scatter from Gate B and gate widths on the order of 2 – 10 µs. The relative delays can be optimized by observing PMT signal in “free run” mode, scanning Gate B delay upwards from a value equal to Gate A delay, and looking for the cutoff where Gate B drops to much lower counts than Gate A. The widths can be optimized by reducing them until signal starts to be
diminished strongly. These widths should be set at the minimum possible to avoid averaging in noise but collect all signal.
The absolute pump-probe delay can be set to any value desired. A good setting is to arrange that at Gate A delay = 0 (i.e., the minimum setting for the triggering electronics), so that the probe pulse actually occurs slightly before the pump pulse. This way the delay can be scanned through zero to obtain a true baseline for any dissociation kinetics scans. This absolute pump-probe delay can be measured by a photodiode each collecting scattered OPO output and the SP YAG scattered light (fed into Channel 1 and 2 of the oscilloscope). These are normally left set up on the laser table so that the delay can be checked at any time by turning on the scope.
The pump-probe delay can be varied via the Infinity laser software. In the Infinity Application Interface window are two variable settings with scroll bars under High- precision Trigger: the coarse and Vernier scales. One count on the Vernier scale
corresponds to approximately 0.5 ns, and the coarse scale is very coarse. It is likely only the Vernier scale will need to be varied. With Gate A delay = 0, the Vernier scale setting can be adjusted while observing the delay on the scope to set the delay for probe preceding pump. Then the absolute values of Gate A and Gate B can be set for the pump-probe delay appropriate for a given experiment.
Spectra Physics YAG 6400 Hz
Trigger Out (sync)
Photon Counter
(SRS) Vic’s box/ 64 Photon Counter(SRS) inverts signalVic’s box
Infinity Infinity 100 Hz Flashlamp trigger
Delay Line 2 Out
Trig In Trig In Trig In Trig In Delay Line 1Out (inverted) Trig In
S.P. YAG set to Rep rate = 0, Fires on every trigger Two possible modes of laser triggering
UV probe pulse
IR pump pulse
A Gate Out Both at 100 Hz, scan delay time (for short [OH] decay)
6400 Hz “comb” of UV pulses over 100 Hz IR pulses
Spectra Physics YAG
Three way switch Two way switch BNC connector “Vic’s Box”
Frequency Dividers See key below Shown for ÷ 64
Delay generator: Either internal via pot knob or external via BNC. Switches toggle ext/int (as shown, both delays int)
3-way switches “amplify” the delay –middle position is smallest delay range, left is medium, right is largest range. For either int / ext delay. Corresponds to channel (top = Ch 2, bottom = Ch 1).
Trigger input
Switch changes polarity of this inputpulse
Channel 2 Out
Channel 1 Out
Outputs.
Left BNC connectors are down polarity, right are up polarity
The four frequency divider switches act like bits.
Here are some configurations and the effect on the signal.
Same orientation as above: up is towards nearest edge of box.
Frequency Dividing ÷2 i.e. 100Hz -> 50 Hz ÷4 ÷8 ÷16 ÷32 etc… ÷64 i.e. 6400 Hz -> 100 Hz
Figure 2-8. “Vic’s box” signal processing electronics used for laser triggering and timing schemes.