Set-Up of the TOFMS for 2D Experiments

The basis o f the experim ental arrangem ent rem ains unaltered (Fig. 2.11). T he TO FM S is an im plem entation o f a tw o-field standard W iley-M cLaren device, w ith a first-order space focus.^

Electron Gun MCP -1850 V y 1 50 ÇI Impedance Matched Transmission Line Pulse Generator Repeller Plate Start -V CFD TDC Amplifier/Discriminator Stop CFD

The ions are form ed by electron im pact, now using a pulsed electron beam (Fig. 2.11), and accelerated out o f the source region by a pulsed voltage applied to the repeller plate. A fter being further accelerated in the second acceleration region, the ions enter the drift region and eventually im pinge on a m ultichannel plate (M CP) detector.

Experimental Arrangement

Ionization of the target m olecules occurs follow ing the interaction of the target gas with a pulsed electron beam . The lens elem ent of the electron gun (Fig. 2.11) is pulsed from an appropriate bias voltage {e.g. -1 5 0 V for 250 eV electrons) to +200 V. The bias voltage is of sufficient m agnitude to stop electrons from travelling down the hypoderm ic needle and reaching the ionization region. Thus during the interpulse period, no ionization occurs. W hen the +200 V is applied to the lens, electrons pass into the ionization region and interact with the target gas. The optimal bias voltage and width o f the voltage pulse applied to the lens to allow the electrons to reach the ionization region (5-40 ns) are evaluated at each electron energy in order to obtain short electron pulses and w ell-resolved spectra. H owever, at low electron energies the electron pulses are tem porally broadened and therefore ionization occurs over a longer tim e period which results in an increase in the widths o f the ion peaks observed in the m ass spectra. Indeed, below 70 eV, it is not possible to produce reliable electron pulses and as a result, no spectra w ere recorded below 70 eV. In order to record adequate spectra at low electron energies, a new electron gun has been designed w hich should enable w ell-defined electron pulses to be produced below 70 eV. This electron gun is currently being com m issioned and prelim inary results indicate that reliable electron pulses are indeed being produced at low electron energies.

The advantage o f pulsing the electron beam is that ionization occurs im m ediately before the application of the ion extraction pulse. As a result, the m ajority o f the discrim ination effects discussed in Section 2.3.1.2 are elim inated, since the trapping o f ions in the space-charge o f the electron beam experienced in the one-dim ensional experim ents will no longer occur. In addition, there is no time for energetic ions to leave the focused volum e and so, in principle, all the ions form ed can be detected.

A fter a set delay (10-20 ns), ions form ed follow ing the interaction o f the pulse of electrons with the target gas are accelerated into the drift region o f the TO FM S by the +400 V extraction pulse applied to the repeller plate. The period between the application of the pulse to the lens elem ent of the electron gun and the extraction pulse is also optim ised for each electron energy, in order to obtain w ell-resolved, narrow peaks in the spectrum . As ionization occurs im m ediately before the application of the extraction pulse, the distribution o f initial positions in the source is small and so

the distribution o f flight tim es for a given ion will also be sm all, thus im proving the m ass resolution of the spectra. < 20 ps > . _ 5-40 ns Pulse to lens of electron gun 10-20 ns 10 ps R epeller plate pulse k 600 ns

>

Fig. 2.12 Schematic diagram of pulse timings for the 2D experimental arrangement o f the TOFMS.

At a set period (600-650 ns) after the application o f the extraction pulse (Fig. 2.12), a signal is sent to the tim e-to-digital converter (LeCroy 3377 TD C) w hich acts as the ‘start’ pulse. The start pulse is sent after the extraction pulse is applied to the repeller plate so any R F noise resulting from the application of the +400 V will not be detected. This start pulse is sent from the pulse generator to the TD C via a LeC roy 3420 constant fraction discrim inator (CFD) to convert the N IM pulse produced by the pulse generator to the E C L pulse required for the operation of the TDC.

Multichannel Plate Operation

The m ode o f operation o f M CP detectors is sim ilar to that of the single channel electron multipliers. The detector assem bly used in these 2D experim ents com prises a pair of plates (Fig. 2.13) with a diam eter o f 40 mm, constructed from special glass through w hich a large num ber of channels of 25 pm diam eter pass. Each channel acts as a channeltron and the output pulse from a pair o f M CPs generally contains 10^ electrons for each input ion.^

Pair of M CPs D rift Tube

C opper A node

-1850 V 50 V V

T o A m plifier/D iscrim inators

The pulse o f electrons from the back o f the M CP assem bly (Fig. 2.13) are collected on a copper anode, constructed from glass/epoxy circuit board, and passed to the pream plifier (x 200) and then to a pair o f discrim inators. The first CFD perform s the true discrim ination and the second CFD is used solely to produce an EC L pulse from the N IM output o f the first CFD. The second CFD also has a adjustable dead time, a tim e interval follow ing the discrim inator output during w hich the CFD cannot trigger again, w hich avoids m ultiple pulsing. F or these experim ents this dead tim e is set at 64 ns.

Recording Spectra

The ions accelerated out o f the source region by the application o f the extraction pulse pass into the drift region and finally im pinge on the M C P detector. Thus, as in the one-dim ensional experim ents, ftof o f an ion betw een the source and the detector is recorded. The set-up o f the TDC is such that once the start pulse has been received, there is a finite period of tim e during w hich the TD C will count up to 32 stop pulses. In the current experim ents this tim e range is set to 10 ps. A fter this tim e period, any ion times recorded are transferred from the TD C to a m em ory (LeCroy 4302), using the FERA (Fast Encoding and R eadout ADC) system , designed for fast conversion of analog inform ation into a digital format. If the TDC received a single stop pulse during the 10 ps ‘w indow ’, an event containing one flight tim e is stored in the mem ory. H ow ever, if the TD C received two stop pulses in the tim e range then a single event containing the two flight tim es is stored in the memory.

The acquisition procedure is repeated until the m em ory is full (I6 K o f data) and the contents are then extracted by the custom -w ritten data acquisition algorithm and transferred via a SCSI port to a PC. H owever, w hile the contents of the m em ory are being dow nloaded, a process which takes about 14 s, any data collected by the TD C is lost. O nce the data contained in the m em ory has been extracted, the data acquisition continues until the cycle o f filling and em ptying the m em ory has been com pleted about 200 times. The data acquisition rate is estim ated to be approxim ately 400 Hz.

The data acquisition algorithm is coded (Visual B asic) so that the events corresponding to single ions and pairs o f ions can be distinguished, thus enabling tim e-of-flight mass spectra and coincidence spectra to be recorded concurrently. U pon extraction o f the data from the memory, the algorithm works out w hich m em ory events contain either one or two ion tim es. If the event contains a single ion flight tim e then the signal is added to the histogram o f the num ber of ions versus Ttof- The com plete histogram , the tim e-of-flight mass spectrum , is displayed on a PC. To avoid confusion, these m ass spectra are term ed ‘singles’ spectra. If the algorithm determ ines that an event contains two ion times, then these flight times are stored. From the resulting list of pairs o f ion flight times, a 2D array is constructed and this array is used to plot the histogram o f num ber o f ion pairs

versus the tw o flight tim es, t\ and ti- This plot of is a 2D coincidence spectrum and, again to avoid confusion, such spectra are term ed ‘pairs’ spectra.

There is, how ever, an inherent com plication associated with the collection and assignm ent of data described above. If an ion form ed from dissociative double ionization is detected within the tim e range but its correlated partner is not, due to the detector efficiency being <1, then this signal is plotted in the singles spectrum. So, undoubtedly there w ill be som e contribution o f fragm ent ions from dissociative m ultiple ionization in the singles spectra. It is not possible to resolve the signals of ions from double ionization plotted in the singles spectrum w hilst data acquisition is in progress, but, as discussed in C hapter 3, the contribution o f m ultiple ionization to the singles spectrum can be determ ined and subtracted m anually.

Operating Parameters

Typical operating pressures in the TO FM S, as recorded by an ion gauge, w ere of the order of 1.3 X 10 "^ Pa. Low operating pressures are used in conjunction w ith the optim al em ission current to give an ion count rate o f the order of 100 counts per second thus avoiding saturation of the detector. Typical operating param eters are listed in Table 2.3.

Table 2.3 Typical operating parameters for the TOFMS used in the 2D experiments to record spectra at 250 eV.

Param eter Typical value

Electron energy 250 eV

Pulsing period 20 ps

Filam ent potential -50 V

Lens potential -150 to +200 V

Lens pulse width 5 ns

Electron needle potential 200 V

Gas needle potential 200 V

R epeller plate voltage 0 to +400 V

O nset o f repeller plate pulse 10-20 ns after electron pulse R epeller plate pulse width 10 ps

D rift tube voltage -1550 V

O nset o f TD C start pulse 600 ns after electron pulse

M C P potential -1850 V

M CP back plate 50 V

CFD dead tim e 64 ns

CFD output width 32 ns

D iscrim inator threshold 100 m V