Dear Mark,
This He+ signal you are observing is probably a result of generating electrons from the laser beam (355 or 266 nm) hitting some of the metal surfaces. If the ionization was from just the gas-laser beam interaction then the signal should be simple, as you were expecting.
How are you directing the VUV/355nm into the mass spectrometer? When I first did this with my TOFMS I used a MgF2 lens and I always observed a helium ion signal, and no matter how well I thought the beam was aligned this ion this would not go away. My solution was to use a waveguide to take the light from the tripling cell to the ionization cell. My waveguide is simply a 6 mm glass tube, touching the front window of the tripling cell to about 1.5 cm from the extraction zone of the TOFMS. The wavelength guide is just a better way of sending the light through the system without hitting metal plates.
Ultimately, you might want to separate the VUV from the 355 nm. This can be done by hitting a MgF2 lens off-axis, so that the VUV and 355 nm spatially diverge. But you will have to effectively dump the 355 nm before it hits any metal plates of the TOFMS.
I have had good success using a waveguide, and the helium signal is orders of magnitude lower than the VUV ion signals. The helium ion signal should turn-off dramatically as the 355 nm energy is turned down. If you can phase-match your tripling mix you should be able to get the same amount of VUV out as you turn down the YAG laser energy by simply adding more gas to the tripling cell. However, when the tripling gas pressure is increased the wavelength range for tripling gets narrower, so it can be very hard making up the correct gas ratio. If you are tripling with a dye laser you can readily work out if you have the correct ratio, but if you only have a YAG laser it is not so easy to work out the correct Xe:Ar ratio.
Regards,
Mark Blitz
-----Original Message-----
From: Molecular dynamics and chemical physics community [mailto:[log in to unmask]] On Behalf Of Mark Sulkes
Sent: 12 June 2018 03:46
To: [log in to unmask]
Subject: anomalous detector response in molecular beam TOF system
Dear All,
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!
Cheers
Mark Sulkes
Tulane University
New Orleans, LA USA
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