I'm seeking the solution to an anomalous response, that popped up from one day to the next, in the dual microchannel plate detector output in a time of flight molecular beam system (Jordan AREF). To generate a time of flight mass waveform, a General Valve gas pulser was pulsed with He containing some seeded nitrotoluene. Pulses of 355 nm passed through a rare gas (Xe/Ar) cell, generating 118 nm, and then into the ion source region. A (sometimes) convenient fact when sufficient residual 355 nm radiation is present is that a He+ signal can be generated. When this is the case for us, it appears at the detector 7 microsec after the laser pulse.
The recorded MCP response as a function of time (relative to a laser triggered photodiode) when the laser pulse is blocked from entering the ion source region is a flat baseline; there is definitely a MCP response when the laser beam is allowed to pass through the ion source region, but not a normal expected one. If the timing of the laser pulse relative to the gas pulse (standard setting) is changed the MCP signal disappears; if the gas pulsing is stopped the signal disappears. Thus, the MCP is responding with current generated by detected ions. (Measurements of MCP current output with an electrometer also indicate functionality.) Here is the problem: One would expect sharp downgoing TOF mass peaks--for instance, at 7 microsec. Instead, there is what looks like a step function integration. Each step occurs where presumably a sharp mass peak should appear (first step at 7 microsec, then additional steps at other presumed actual mass peaks).
The standard laser alignment was checked. Instead of 355 nm, 266 nm pulses were sent through another port, and again a step function response was seen. Cables were replaced. Another digitizer was used. Both digitizer and DC supplies for the molecular beam were plugged into 120 VAC that went through an isolation transformer. The system is a fairly standard one where we have never observed an anomalous response like this before.
I would gladly accept the embarrassment of having a really dumb oversight pointed out to me if anyone can suggest possible causes, further troubleshooting steps, or possible remedies!
New Orleans, LA USA
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