Time resolved photoionization mass spectrometry in the millisecond range

International Journal of Mass Spectrometry and Ion Processes, 6 ~ (1984) 99--105

99

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

TIME RESOLVEDPHOTOIONIZATION MASSSPECTROMETRY IN THE MILLISECOND RANGE C. LIFSHITZ and Y. MALINOVICH Dept. of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904 (Israel)

ABSTRACT A photoionization mass spectrometer operating in a time resolved manner, from the microsecond to the millisecond range, has been constructed (ref. l ) . A Hinteregger VUV l i g h t source is pulsed and the photoions produced are trapped in a Cylindrical Ion Trap (CIT). Time resolved ionization efficiency curves are obtained. Several molecules which demonstrate large 'kinetic shifts' (ref. 2) have been studied. Dissociation rate coefficients of ~ l sec-1 or lower have been observed by using extended signal averaging techniques. INTRODUCTION The 'kinetic s h i f t ' is defined as the excess energy required to produce detectable dissociation of a polyatomic ion in lO"s seconds (ref. 3). I t may introduce very appreciable errors in appearance enercLV determinations (ref. 4), since an ion may have enough energy to dissociate but not have enough time to do so. We have constructed and tested recently (ref. 1,2,5) a photoionization mass spectrometer which performs time-resolved experiments. I t f u l f i l s the obvious need for an experimental approach for evaluating kinetic shifts (ref. 6), namely measuring the s h i f t of the ionization efficiency curve along the energy axis as the residence time of the ion is varied. I t combines the excellent energy resolution of photon impact with an extension of the time scale from microseconds to milliseconds. The central part of the instrument is a Cylindrical Ion Trap (CIT) similar to the one employed previously (ref. 7) for electron impact work. Some experimental characteristics of the instrument w i l l be presented, as w i l l be time-resolved ionization efficiency curves and appearance energies for ion storage times of up to 500 ~s. EXPERIMENTAL Photoionization is induced by a pulsed VUV l i g h t source (a Hinteregger discharge in hydrogen producing the many line spectrum) and the photoions are trapped by the CIT. The instrument is shown schematically in Fig. I. Photoions are ejected into the quadrupole n~ss f i l t e r by a drawout pulse following a

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Fig. I. Time-resolved photoionization mass spectrometer (TPIMS). The VUV source is a Hinteregger hydrogen discharge lamp; the monochromator is a McPherson model 225 l-m, normal-incidence instrument, the quadrupole mass spectrometer is an EAI QUADI l l 0 , the computer is an Industrial Micro Systems, IMS 5000, micro-computer; PMT- photomultiplier; CEM- channeltron electron multiplier; PG-pulse generator. variable delay time, and counted only during the ejection pulse. Ions can be stored from microseconds to milliseconds as is shown in Fig. 2. The characteristic dimensions of the CIT employed are r I = 2 cm and Zl= 1.5 cm, (see ref. 7 for definitions of r I and Zl). Typical operating conditions are as follows: the radiofrequency of the potential applied to the cylindrical barrel electrode is 0.5 MHz (~/2x) and the peak to peak voltage between 400800 V (depending upon the ion masses which are trapped); the cylindrical electrode is biased by a 0 to +20 V DC potential for optimum operation. The planar end-cap electrodes are earthed and ions are ejected after a pre-determined storage time by means of a -30 to -60 V pulse (width 20-50 ~s) applied from a pulse generator PG2 to the end cap nearest to the mass f i l t e r . The number of ions stored within the device, as indicated by the number collected following pulsed ejection from the trap after a given storage time, exhibits a maximum as a function of RF voltage, the position of which is mass dependent (Fig. 3).

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Fig. 3. Observed ion count rates as a function of RF voltage for parent (CBH8, m/z= I04) and fragmento(C6H6+, m/z = 78) ions from cyclo-octatetraene (COT). Wavelength = x~ (1216 A); DC bias = +20 V. light intensity emerging from the exit s l i t of the monochromator and entering the CIT is measured with a Pyrex window coated with sodium salicylate and a photomultiplier. A voltage to frequency (V/F) converter provides a digital l i g h t signal from the analog one obtained from a Vibrating Reed electrometer. Counters are employed to give the ion and l i g h t signals, respectively, at each wavelength setting, and these are stored in the computer. The data (ion and l i g h t intensity counts and PIE) are obtained on the computer screen in real time as is the PIE curve which is accumulated on an oscilloscope. Each computer-controlled experiment is made up of many repetitive cycles. In each cycle a l l the wavelengths ( i . e . , photon energy points) are scanned for a l l the relevant ions and the time intervals are chosen so as to obtain a nearly constant signal-to-noise ratio, at each energy point. A typical cycle takes between 1500-2000 seconds ensuring minimal errors due to slowly varying parameters (e.g., sample pressure) during a cycle.

103

RESULTS AND DISCUSSION Fig. 4 represents the photoionization efficiency (PIE) curve from a computer controlled experiment for the fragment ion (C6H6+) from cyclo-octatetraene (COT) at a storage time of 500 ~s in the CIT.

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ENERGY Fig. 4. Photoionization efficiency (PIE, ion counts divided by l i g h t intens i t y ) as a function of photon energy (in eV), for the C6H6+ fragment ion from COT at a storage time of 500 ~s. Energy resolution near threshold, ~0.035 eV. Traeger and coworkers (ref. 9,11) have employed a linear extrapolation method for appearance energy (AE) determinations. We have adopted a "vanishing current" technique which leads to lower AE values because of contributions from the ionization and subsequent fragmentation of thermally excited precursor molecules. In spite of this we feel that this technique is less equivocal and more suitable for kinetic s h i f t determinations, i f employed consistently for experiments run at different storage times under constant detection sensit i v i t y . This technique (ref. 12) is also the one which we have adopted (ref. 13-14) for our time-resolved electron impact (El) experiments and allows therefore consistent intercomparison of PI and El appearance energies, for the

104

same processes, to be made. Table I summarizes AE data for several molecules demonstrating large kinetic shifts. Most of the values obtained in the present study are considerably lower than the ones previously obtained (ref. 2). This is understandable since i f the specific rate coefficient is k(E)~ l sec-1, as i t is for the pyridine ion reaction at threshold, extending the reaction time from t= 5xlO-6 to 5xlO"4 seconds, increases the fractional abundance of the fragment ion from 5xlO-6 to 5xlO"4. TABLE l Time-resolved appearance energies, AE by photoionization at 298°K

Molecule

Reaction

AE/eV

Time/us

lodobenzene

C6HsI t ÷ C6Hs+ + I.

10.55± 0.I

500

Pyridine

CsHsNt + C4HJ + HCN

11.84± 0.05

500

Aniline

C6HsNH2 ÷ C5H6t + HNC

l l . 3 ±O.l

500

Phenol

C6HsOHt ÷ CsH6t + CO

11.4 ± 0.I

70

COT

CBHBt ÷ C6H6t + C2H2

9.4 ± 0.05

500

Ethylbenzene

C6HsC2Hs t ÷ C7H7+ + CH3.

9.9 ± O.l

450

Some of the fragment PIE curves which we have obtained at long storage times demonstrate a sharp increase within several tenths of an eV above the f i r s t onset. This is true for C7H7+ from ethyl-benzene and for C4H4t from pyridine. These higher onsets are ascribed to isomeric structures. Thus, the benzyl cation has an onset, which is 0.9±0.I eV higher than tropylium (from ethylbenzene). An isomeric C~H4t structure has an onset 0.5±0.1 eV higher than themost stable C4H4t structure from pyridine. The l a t t e r onset is in agreement with the estimated difference in heats of formation of methylenecyclopropenium and linear C4H4t (ref. 15). I t has been demonstrated previously (ref. 16,17) that two dist i n c t C4H4t structures are formed, from benzene as well as from pyridine, whose relative abundance is energy dependent. Further work on time-resolved breakdown curves is now in progress (ref. 18). This w i l l lead to more detailed information on activation parameters for unimolecular dissociations of polyatomic cations at near threshold energies.

105 ACKNOWLEDGEMENT This research was supported by a grant from the United States-lsrael Binational Science Foundation (BSF), Jerusalem, Israel. Dr. A.C. Parr serves as the American Cooperative Investigator for this grant. The authors would like to thank A. Moss, M. Peres and T. Peres for the design and construction of the computer interface and control systems and M. Goldenberg and G. Hase who werepartly involved in some of the experiments.

REFERENCES 1 C. L i f s h i t z , M. Goldenberg, Y. Malinovich and M. Peres, Org. Mass Spectrom. 17 (1982) 453-455. 2 C. L i f s h i t z , Mass Spectrom. Rev., 1 (1982) 309-348. 3 L. Friedman, F.A. Long and M. Wolfsberg, J. Chem. Phys. 26 (1957) 714-715. 4 W.A. Chupka, J. Chem. Phys. 30 (1959) 191-211. 5 C. Lifshitz, M. Goldenberg, Y. Malinovich and M. Peres, Int. J. Mass Spectrom. lon Phys. 46 (1983) 269-272, 6 W.A. Chupka and J. Berkowitz, J. Chem. Phys. 32 (1960) 1546-1553. 7 R.E. Mather, R.M. Waldren, J.F.J. Todd and R.E. March, Int. J. Mass Spectrom. lon Phys. 33 (1980) 201-230. 8 G. Lawson and J.F.J. Todd, Anal. Chem. 49 (1977) 1619-1622. 9 J.C. Traeger and R.G. McLoughlin, Int. J. Mass Spectrom. lon Phys. 27 (1978) 319-333. I0 E.J. Darland, D.M. Rider, F.P. Tully, C.G. Enke and G.E. Leroi, Int. J. Mass Spectrom. lon Phys. 34 (1980) 175-192. II R.G. McLoughlin, J.D. Morrison and J.C. Traeger, Org. Mass Spectrom. 14 (1979) 104-108; J.C. Traeger and R.G. McLoughlin, J. Am. Chem. Soc. 103 (1981) 3647-3652. 12 P.C. Burgers and J.L. Holmes, Org. Mass Spectrom. 17 (1982) 123-126. 13 S. Gefen and C. L i f s h i t z , "Time Dependent Mass Spectra and Breakdown Graphs. V. The Kinetic Shift in lodobenzene; A Time-Resolved Electron Impact Study", Int. J. Mass Spectrom. lon Phys., submitted. 14 C. L i f s h i t z and S. Gefen, "Time-Dependent Mass Spectra and Breakdown Graphs. The Kinetic Shift in Phenol", Org. Mass Spectrom., in press. 15 H.M. Rosenstock, J. Dannacher and J.F. Liebman, Radiat. Phys. Chem. 20 (1982) 7-28. 16 C. L i f s h i t z , D. Gibson, K. Levsen and I. Dotan, Int. J. Mass Spectrom. lon Phys. 40 (1981) 157-165. 17 P. Ausloos, J. Am. Chem. Soc. 103 (1981) 3931-3932. 18 C. L i f s h i t z and Y. Malinovich, Proc. 7th Int. ~nf. VUV, Annals Isr. Phys. Soc. 6 (1983) 243-245.