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Resonance enhanced laser ionisation mass spectrometry of four aromatic molecules
53
Chemical Physics 56 (1981) 53-61 North-Holland Pubiishing Compxty
RESONANCE ENHANCED LASER IONISATION MASS SPECTROMETRY OF FOUR AROMATIC
MOLECULES
Charles T. RETTNER and John H. BROPHY ** MassachusettsInstifuteof Technology, Cambridge, &fa.szuchucetts0.2139. USA l
Received 12 September
1980
Multiphoton idnisation and fraflentation of aniline, benzene, N,Nilimethyl and 2,4dimethyl aniiine has been studied by laser ionisation mass spectrometry under collision free conditions. All four molecules undergo efticient resonanceenhanced w-photon ionisation (R’LPI) with relatively low laser intensities at h = 266 run producing the parent ion almost exclusively. At higher intensities, higher order processes compete producinp extensive fmgmentation. At 266 nm, all the major fragment ions are produced by R3Pl. For aniline excited at 294 run, energetic considerations suggest R4PI formation of fragments with differences in fragmentation between 266 and 294 nm reflecting the differing orders and energies above threshold of the competing processes. Comparison of R2PI efficiencies in aniline and benzene shows that the cross sections for ionisation of the resonant intermediate lBz excited state iF. both molecules are approximately quaI and independent of wavelength in the range 250-300 nm.
1. Introduction The very high proton fluxes generated by lasers have greatly facilitated the study of multiple and multiphoton processes. Initial experiments in the field of multiphoton ionisation (MPI) were concerned with the physics of the phenomenon [ i,Z] but the increasing availability of high peak power lasers has been accompanied by a rapid growth and divergence of the MPI literature (for recent reviews see ref. [3]). Aiongside a continuing interest in the basic MPI process, various workers have stressed spectroscopic [4,5], mass spectrometric [6-81, analytical [9-l 11 and diagnostic [12,13] applications of the technique_ Many molecules have been studied, notably NO, I3 and benzene, and a variety of conditions employed. Most studies have been made on bulk gases, but molecular beams [6,12,13] artd even liquids [ 141 have also beeti employed; Detectors range from s$npIe,
* To-whom all correw.n&nce rho&i be addressed, at :, Dep.aent of C&mistry, Stanford University, Stanford, California 94305, USA; ‘*&k&s&t address: Department of Cl&&try, lJ&&ity of Biiingham, Edgbxton, Birmii&k B15~2l%;‘U~IC.
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two-plate cells to mass spectrometers. When iorisation occurs via a real intermediate excited state, reached by a coherent m-photon excita-. tion, the process is resonance-enhanced (REMPI). The intermediate state then absorbs n further protons to the continuum, ionising the molecule. For m = n = 1, resonance-enhanced two-photon ionization (WI) results and very efficient ionisation may be obtained with comparatively weak lasers. In a recent study [15] it was shown that aniline readily undergoes EUPI when pumped in the region of its 1Bs -‘A, transition at wavelengths between 2!30300 nm. Experiments in molecular beams and bulk gas samples showed the process to be highly efficient and the technique of lU.PI was demonstrated as a
means of monitoring trace amounts of aniline in atmospheric air [9]. Other workers have since confumed these fmdings. Dietz et al. [ 131 have recently reported very similar results using a jet-cooled aniline sample, and Frueholz et al: [i l] have applied the R2PI technique to the detection of naphthaIene vapour, arriving at a similar estimate for ultimate detection limits. In the previous experiments with aniline, the relatively 1oWlaser i&&ties limited the overall order Publiig
ampay
..
54
CT. Rettner, J.H. Btvphy j&fuss spectrometry
of the ionisation process and mass analysis of the ionic products showed almost exclusive formation of the parent ion by R2PI. At higher laser intensi:ies, however, higher order processes may compete with R2Pl and other relaxation rates to produce fragment ions. Here we report further details of the laser ionisation and fragmentation of aniline together with results for two isomeric dim&y1 anilines, N,Ndimethyl and 2,4dimethyl aniline. We also report preliminary results for benzene, complementing other similar studies of this molecule at different wavelengths [6,16,17].
’
the Iascr beam power, the sample being introduced instead as a bulk gas via a needle valve. Also, two different lasers were employed. All experiments involved the use of a Quanta-Ray (DCR) Nd:YAG laser;equipped for the generation and separation of the second (532 nm), third (355 nm) and fourth (266 nm) harmonics. In a series of experiments with aniline, the second harmonic was used to pump a Molectron Rhodamine 6G dye Iaser. On doubling with a KDP crystal, this gave wavelengths tunable in the 290-300 run range with a5 kW peak power at a repetition rate of 10 Hz. Several experiments were carried out with the dye laser, but most studies employed the Nd:YAG fourth harmonic directly. This was obtained with an ~15 ns pulse duration at pulse energies in the range 0.03-25 mJ, again at 10 Hz. Over this range of pulse energies, the pulse duration was observed to be invariant, as was the mode structure of the output beam. Hence average power measurements, taken on a Scientech 362 power meter, were used to estimate peak laser powers.
2. Experimental A schematic of the apparatus is given in fig. 1.
This differs somewhat from that employed previously [IS]. Most notably, the moIecular beam source chamber was removed to allow direct monitoring
of aromatic molecules
of
7
au40
IRISES
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Fig. 1. Scheniatic of the laser ionj&t
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CT. Retmer,
J.H. Bmphy
/Mass spectmmerry of aromatic moiecdes
To check the linearity of the power meter and to observe the laser pulse duration, a portion of the beam was split off with a quartz plate for detection on an RCA lP28 photomultiplier. Two quartz lenses of 5 cm focal length, held on XYZ translators with a separation of approximately 10 cm, served to “collimate” the laser beam and also to provide fiie adjustment of the beam direction. A retractable 30 cm focal length quartz lens was employed in some experiments to bring the beam to a focus at the ion source. Two irises secured to the frame of the mass spectrometer housing served 2s an aid to alignment and to further collimate the beam. The beam was passed via a quartz window through the ionisation region of an EAI 250 quadrupole mass spectrometer. This instrument allows a clean passage through the ionisation region without modification. With the collimating irises fully open the (low-power) beam couId be imaged after passing through this region and positioned so as to be midway between any shadow-casting components of the ion source.
100
1a.’
’
’
’
’
+100
ni
55
On closing the irises, simple line-of-sight observation revealed a clear passage through the Centre of the ionisation region. This alignment is important, since photoe!ectrons are produced when high-power lasers strike metal surfaces, producing electron impact ionisation of the sample. The wavelength sensitivity and absence of air peaks in the mass spectra provide further evidence that this was not occurring. Signals were taken directly from the mass spectrometer electron inultiplier without preamplification and fed either to a Keithly electrometer or to a boxcar signal averager and, in both cases, displayed on a chart recorder. Maximum time-averaged output currents corresponded to u10-8 A. The samples (reagent grade) were used without further distillation, but were subjected to three freeze-thaw outgassing cycles before use and their purity checked with conventional electron impact mass spectrometry. The base pressure in the apparatus was heId at S.5 X lo-* Torr by a S-inch diffusion pump baffled with a liquid nitrogen cold trap. Maximum sample pressures were ==10M5 Torr. Although somewhat high for ion work, other workers have found 1231 ion coilision effects to be negligible under similar conditions.
3. Results
3.1. Atlilirze At moderate laser intensities in the region of the O”,band of the ‘B2--‘AI transition of aniline (293.8 run) very soft but efficient ionisation is achieved. The laser ion&&ion mass spectrum consists of a single parent ion peak at M/e = 93 [ 13,151. As the laser intensity is increased to 20.1 hlW cmw2,however,
IO 208040808076’8090
We. Fig. 2. hIzss.spectra
&the ion fragments produced by laser ionistion of aniliie’excited via’its.‘Bzl’Al transitioa’(a) Excitation of the 0: band at 293.8 nm with a laser intensity -==OS.hfW&II-~; @) excitation at 266 Mlwith B k&r intensity ~0.2 MWcm-2:~(cj’cx~tationst.266.nm With~l~r ix?te@ty’=l.GW cmS2. Notice’the z&l? char& in &gram (a) M/e:= 93 has been divided by 100; and’in (by&z =93 .. anhM/i =.66 have &eh divided by 1000 and’iO~respect&ely.; ..’ : : _‘. -:
., ... .
._
: ‘.
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fragmentation becomes detectable. The laser ionisation (LI) mass spectrum of aniline at 293.8 nm is shown in fig. 2-a. Signal strengths permitted only crude mass resolution (2-3 maxs units), revealing peakscentred at Mje = 93,78,66,52,39 and 28. AS well as the p=ent ion [13,15], the intensity of the second strongest p&k (at M/e - 66) Was also found
to follow the on&photqn absorption spectrum. No measurements w&k mid! on the qther pe+s. The : fi&nentation pafi&ti found to be relatively ‘insensitive to~sn@l wavelength changes r for example, excitation at 294.2 nm produced a sir@ar mass spec-
CT. Retmer. J.H. Bmphy j
56
Massspectmn;etiy o:ammntic molecules
trum. In order to test the effect of a huge wavelength change, the fourth harmonic of the Nd:YAG laser was used directly. This beam, largely due to its superior fluence, was found to be much more suitable for detailed study of the power dependences of the fragmentation pattern. LI mass spectra for excitation at 266 nm employing two extreme beam intensities are given in figs. 2b and 2c, respectively. At the highest intensity, fig. 2c, in addition to the major mass peaks listed above, fragment ions appear at M/e=12and 14. Fig. 3 displays the power dependences of some of the more prominent peaks, and their limiting intensity dependence-s are given in table 1. Although power dependences were taken with the same (low) resolution, at higher laser powers signals were sufticiently strong to allow unit maSS resolution_ In fig. 4 a detail of the mass scan is shown forfif/e = 73-80. For comparison the 70 eV electron impact ma= spectrum recorded in the same sample is indicated in the figure.
=-Parent
Table 1 Limiting intensity dependences
log (signd) a) A log (ktensity)
MolecuIe
A
aniline
93 78 66 52 39 28
2.0 2.4 3.0 3.0 3.25 3.1
f 0.05 2 0.2 b 0.1 f 0.1 c 0.1 + 0.1
2,4dimethyl aniline
121 106 77 42
2.0 3.0 2.9 2.9
i 0.05 f 0.1 t 0.1 L 0.1
N,NdimethyI aniline
121 105 79 41
2.05 2.8 3.2 2.8
+ 0.05 = 0.2 f 0.2 t 0.2
benzene
78
1.95 ? 0.1
a) Obtained from least squares analysis of data for a range of low laser intensities yieIding linear plots.
Ion
A-Mass 66 +-Mass 39 W-Mass 28 ~-Mass I
78
I
lci’
#O 10
IO2 103
Laser. lntensuy/MW cme2 . Fig 3. &rent and fmgment ion yield curves from the laser: : io&tiod of aniline at.266 nm as a’funckon 0: the Ia&. ‘.inter&. 15,e line& plots at I&v interisitiiwer~ dbtained _:~ .,,u!itha_coiIitqatedbeam (see t&t), whks tie c&es at .: -high pow& correspond to a focused beam. Only.iepreseti-’ ; .. M&q data points and enor bars are dig&d foi clarity_ :
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I 73
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I 78-n
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F&P. &it mass rklution’df the &up of.ionit’&nents belsknM/e 2 73,and 80 prdduced in the lker~ioniktion of anilineat A A.266 rq_,Fbr cqmp+n the electron impact ~, masz qkctrum re+ded in the same s+pIe,~k’&&c;ited_b~’ ‘.
CT. Rettner. J.H. Brophy /Mau spectrometry of aromatic molecules
No ionisation could be obtained with the green 532 nm second hamkmic up to the maximum intensities available (=I00 GW cm-*). This is in keeping with the findings of Dietz et al. [ 131. A similar negative result was obtained with third harmonic at 35.5 nm. 20
3.2. 2,4- and N,Ndimerhyl aniline
30
40
50
60
70
80
”
We
Ft. 6. Laser ionisation rna~ spectra for benzene for a laser Laser ionisation signals from these two compounds were found to be vexy similar in magnitude to those of aniline. Fig. 5 shows LI mass spectra for these two compounds at 2-3 mass unit resolution, taken with the fourth harmonic of the Nd:YAG at 266 nm. Power dependences for the major peaks over a range of low laser intensities are given in table 1. Again, the second and third harmonics failed to produce ionisation.
intensity ==l WV cm-* at 266 nm. Note the scale change for the parent ion peak, which is divided by 100.
-120. This large reduction in signal made estimation of the limiting low power intensity dependences impractical for all but the parent ion, which gave a slope of 1.95 + 0.1, confirming the ionisation as a two-photon process.
3.3. Benzene
4. DiscuSion
An LI mass spectrum, again taken with 2-3 mass unit resolution using the fourth harmonic of the Nd: YAC, is shown in fig. 6. Electron impact and LI mass spectra were taken for an aniline-benzene mixture, from which it was found that at 266 nm the R2PI process is more efficient for aniline by a factor of
4.1. Aniline The highly efficient laser ionisation of aniline when pumped in the region of its ’ BZ-‘A, transition is again demonstrated. It may be concluded that the second photon excites the aniline to an autoionising state which is comparatively slow to predissociate into neutral species [ 131. Two photons at 294 nm correspond to 816 kJ mol-’ compared to the ionisa-
z 1100
s~laC&k&HCN
3looO
-Fig. 5. +ser ion&ion III= _$~eCtrsfoi (a) 2,4dimethyl an~~~e-~d~@).N~Ndimethyl anilinp for a laser intensity. zO.2 hW~cm~*,ati66~nm. The regive inteNity bf the +A aiM/t+ Y 93 ir;(& @&I from e+erimckt tb expe&ent z&d: is att&&l to a&& &pu;ity.‘Note Uk. kale d&&s ftir .‘. theparent ionpeaks;~ksea~dividedby.IOO, .‘I .. :,- :
I
,
Fig. 7. Schematic representation of the energetics of ionintion md fragmentation of aniline.Theheats of formation of .repRsentative pr.&ucts xe shown fx comparison with_the &erg) corresponding t+.bsbrption of 23nd 3 photons df -. laser~t~t266and294nm.
58
CT. Retmer. J.H. Brophy /Man spectrometry of aromatic nwtecules
tion potential of aniline, which is 740 k3 mol-’ [I 81. The remaining energy is insufficient to fragment the parent ion, which explains the “soft” nature of the ionisation at low laser intensities. At higher intensities, the probability for absorbing further photons becomes significant. The energy level diagram shown in fig. 7 reveals that even three photons at 294 nm may not be enough for significant fragmentation, particuIarly since there are almost certainly activation barriers for these rearrangement-dissociation processes. Fragmentation to the cyclopentadienyl ion plus HCN would be possible, but the more energetic linear &Hz ion was found to be favoured in a conventional eiectrcn impact mass spectrometric study [19] of the decomposition of C6H5NH:. The fact that the ionisation efficiency of the “C:” mass 66, peak also followed the one-photon absorption spectrum stiggests that the same one-photon resonance enhancement applies to the generation of the fragment ions. Thus we conclude that at least four pho-
tons are required to produce the fragment ions shown in fig. 2a, i.e., m = I, n B 3. Whereas excitation at 293.8 nm was known to pump the 0: band of the ‘BZ-‘AI transition, the exact transition pumped by the 266 nm beam is not known. However, the resonant state is almost certainly the same IBz state [20], but with considerably more vibrational energy:. AIthougb a small wavelength change to 294.2 nm did not produce a significantly different fragmentation pattern, the larger change to 266 nm had a substantial, but not drastic effect. Comparison of figs. 2a and 2b requires some caution, since the two spectra were not recorded with identical laser beam pro?des or power. The gross differences observedare however attributed to the wavelength change. Two such differences arc apparent: (i) the degree of fragmentation is greater at the shorter wavelength, and (ii) the mass 66 peak represents a much larger fraction of the fragment ions at this wavelength_ We will return to this point later, after considering the meaning of the power dependences observed for excitation at 266 run, fig. 3. Multiple and mtitiphoton ionisation processes [ZIP can be described by a rate equation approach ]3,17, 22,23]_ By model& the process as a many-!evel sysi tern, exprtions for the various ion populations may be obtained. Ln a previous study [15].we used such an approach to obtain an estimate for the photoionisa-
tion absorption cross section for the *Ba state and found it to be =3.5 times larger than that for the initial ‘BZ-‘A, transition_ Others have recently attempted to ratio&se ion yield curves with this approach [ 17,231. In the most comprehensive treatment to date, Robin and co-workers [23] derive expressions for the populations of fragment ions under a variety of conditions. What emerges from these treatments is that in the limit of low-intensity fragment ion yields should increase as Imen> where the ion is produced following the absorption of m +rt photons. In most cases signals do not persist to this limit. The suitability of aniline for this type of study is again emphasised. The Linear slopes exhibited at !ow laser intensities, fig. 3, are taken as indicative of the number of photons required to yield the respective fragment ions. It is seen that, apart from the parent ion and the peak at mass 77, the other major fragments exhibit cubic intensity dependences consistent with formation via a three-photon process. The dependence of the mass 77 peak was found to be non-integer even at the lowest intensities reached. In the absence of an alternative explanation, we attribute this anomaly to an impurity. For example, a trace quantity of benzene would give a twophoton dependence to be added to a three-photon contribution from a “‘genuine” aniline peak. The very high efficiency of R2FI processes makes such interference a real problem when looking at higher-order frngment ion peaks. The gross differences in the laser ionisation mass spectra for exc.itation at 293.8 nm and 266 nm may now be rationalised. We believe that excitation at both wavelengths leads to two-photon formation of the parent ion. At 266 nm fragmentation occurs to yield other ions, predominantly via an overall threephoton process. These same fragments require four photons upon excitation at 293.8 nm. We believe that this difference in order for the process accounts for the reduced fragmentation at the longer wavelength. The differing relative importance of the mass 66 fragment may also be rational&d in terms ,of the differing excergicities of the various fragmentation &annels. We attribute the has 66 peeto the.linear &Hz ion. This species, together with its companion .. product, HCN,.represents the-lowest ener& of +rry of the fragmentation products and most likely results
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C.T. Retuner, J.H. Brophy /&iass spectrometry
of three-photon fragmentation at 266 MI, the small ener,v differences between the various dissociation paths will be important since the three-photon energy exceeds the fragmentation threshold by only ~150 kJ mol-r . This leads to predominant formation of the lowest energy products, specifically the &Hz ion plus neutral HCN. Four-photon excitation at 293.8 nm, however, exceeds threshold by -30 W mol-’ , and the various fragmentation rates become more nearly equal. The other major fragment ions are (tentatively) assigned to the formulae ChH; (mass 77); C4H$ (52); CaH; (39); and H,CN’ (28), all of which are major peaks in the electron impact spectrum of aniline. As the laser intensity is increased, the integer dependences of the ion yields on laser intensity are lost as saturation effects, ion photofragmentation and multi-path processes become important. This leads to the appearance of new peaks in the mass spectrum, e.g. at M/e = 12 and 14 in fig. 2c, and to a gradual equahsation of peak heights. The same general trends have been reported by others [6,17,23,24], some of whom have attempted a fit to the rate expressions mentioned above. We do not feel that such a treatment would be fruitful here, due to the complexity of the system. Specifically, there are many unknowns, such as radiative rates, adsorption cross sections, the locations of sink states, etc. Furthermore, the exact geometrical factors pertinent to our system as saturation is approached with a focused beam are, as yet, undetermined. 4.2. 2,4-and
N,Ndimethyl
aniline
In a study of the molecules naphthalene and azulene, Lubman et al. [24] demonstrated that LI mass spectrometry can readily distinguish isomers of large organic molecules. The spectra shown in fig. 5 for the two diiethyl anilines considered here support
this finding. Although not explored here, the R2PI spectra of the two molecules would also show differences imposed by their differing one-photon absorption spectra. As su&, the LI mass spectrometry technique offers an extra dimension to conventional electron irnpact,mass spectiometry. In fairness, the fragmentation patte@ observed here show no more signitkant-differences than the converitional mass spectra 1251. ‘khe fact that ions are obt&ed at 266 . :’
of arornbc mokcules
59
run and not at 355 run or 532 run does give further information, indicative of the chemical and spectra-’ scopic sirniiarities of the isomers. The ionisation potentials for the 2,4 and N,Ndimethyl aniline are 714 kJ mol-’ and 685 kJ mol-’ , respectively, compared to that of 740 kJ mol-’ for aniline [ 181. Hence, the energetics of the various processes are essentially identical to that described for aniline. The power dependences given in table 1 for the major fragment ions of both isomers indicate three-photon production of these species. For 2,4 dimethyl aniline, these are assigned as [CHs - &HsNHs]+ (106); CsH: (77); and CHaCHN* (42). Likewise, for N,Ndiiethyl aniline
we have CeHs.NCH:
(105); CeH: (79); and
CHsCN’ (41). As with aniline, the fragmentation is qualitatively similar to that found in electron impact mass spectra [25]. Whereas the parent aniline ion readily loses HCN, both dimethyl aniline parent ions display a propensity far loss of a methyl group [26]. 4.3. Benzene As a prototype molecule, the MPI of benzene has been (comparatively) well-studied, beginning with the pioneering work of Johnson [27] and more recently taking the form of LI mass spectrometric studies. Such studies have been reported by three groups. Zandee and Bernstein [6,28] investigated the fragmentation of benzene in a molecular beam using a near-W dye laser excitation source focused to intensities of up to 2 X 10” W cm-‘. Since they were exploiting an initial two-photon resonance, such high powers were necessary to give adequate signals but resulted in considerable fragmentation. Boesl et al. [ 161 demonstrated R2PI of benzene in a molecular beam using much smaller laser intensities (~1 MW cm-*). Consequently, they observed only the parent ion (cf. refs. [13,153 for aniline)_ Most recently, Reilly and Kompa [17] employed KrF (248.5 nm) and ArF (193 nm) laser lines with pulse
energies of up to 9 ml, using a 10 cm focal length lens, from which we estimate intensities >1CKlMw cm-= . They observed increasing fragmentation with laser intensity that described The results other studies,
in a qualitatively similar fashion to above for aniline. reported here are consistent with these The fragmentation pattern shown in
60
CT. Rettner,. 3.H. Brvphy /Muss specnometry of aromatic nwleadPs’
fig. 6 displays Cs,C4, C5 and C6 peaks. Energetic arguments identical to those applied above, considering which species are energetically accessible as well as dynamically feasible, suggested the following assignments: CsH; (39); C4Ht (50); CsH; (63); and CsHz (78). AU of these are major fragments in the electron impact spectrum and were observed by Reilly and Kompa [ 173. Although it proved impractical to measure the limiting intensity dependences, energetic considerations [ 181 similar to those outlined for aniliie suggest that at least four photons would be necessary to produce the fragments observed_ This difference from the anilines is largely the result of the increased ionisation potential, which is =I50 kJ mol-* higher than for aniline, for example. Th facto: of W-120difference in the R2PI efficiencies for benzene and aniline was observed at low laser intensitiesand, therefore, approximates to the ratio olhua.4/u1n~au, where the subscripts 1 and 2 refer to absorption from the ground and resonant states, and A and B to aniline and benzene, respcctively. We estimate uIA (266 nm) to be slightly smaller than ul~ (293.8 nm) [IS], say 7 X lo-‘* cm’. Literature data-for benzene [29] indicates or8 = 5 X lO”e- cm*. Thus (JlAialB = 140, suggesting that @&,= f&B. This is consistent with previous observations if we assume that the wavelength dependence of these cios sections is slight. u;?B (248.5 nm) has been estimated as 3.4 X 10-l’ cm’ [ I7], which is close to the value of 3.5 X lo-” cm* that we reported for oaA (293.8 nm) [.I51_
5. Conclusions hiiine, benzene and the isomeric substituted &es, N,N.and 2,4&nethyl-aniline, have been. -shown to undergo efficient, resonanceenhanced; : multiphoton ionisation when excited via their ‘Et1_4I transitions in the W_,At EJ’ghla&r $te&itjr, extensive~frag&nti?& o&irk prodking & of..the. io&fiagmen*ts obs&ed ir&&e&onal electron I
As might be e?rpected for absorption http a largely structurelesscontinuum, the cross se&ions for onephoton ionisation of the resonant in&-mediate. ... excite-d states of benzene~aud aniline are approxi- s mately equal and independent of wavelengthabove. threshotd.
Acknowledgement We are grateful to J.L. Kinsey and R-W. Field who made facilities available in the spectroscopic.laboratory at Massachusetts Lnstitute of Technology; We also acknowledge partial support of this work by theNational Science Fou&ation and the US Office of Naval Research.
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Tsubomun ami S. Nagakw, BuiL Chem.’ (1964) 1336. and J.H. Eberly, Pbys. Rev. A14 (1976) and P.M. Johnson, J. Cbem. Phys. 6X
(19%) 3644; Chem. P&s. 46 (1980) 263. [23] G.J. Finick,T.S. EicheIbergeI IV, B.A. Heath and M.B.Robin, J. Cbem. Phyr 72 [iSSO) 557 1. [24] D-M. Lubmarx, R. Naamanand R.N. Zare, J. Chem. Physi 72 (1980) 3034. [25] Eight peak index of mass spectm, Znd~ed. (Mm Spectrometry Data Centre, AWRE, Aldermasion, 1974). j26j S. Hammerurn and P. Wolkoff, Advan. Mass Spectrom.
7B f1978) 1312. /27] P.M. Johnson, J. &em. Phys. 62 (1975) 4562;64 (1976) 4143. {28] L. Zandee and R-B. Be@stein, J. Cbem. Phys 7111979) 1359. [29l W_5. Potts Jr., J. Ckem. Phys 23 (195.5) 73; J.H. Callomon, T.hi. Dunn and I.M. MilIs, Phil. Trans Roy. Sac. (London) 259A (lS66) 499.
Chemical Physics 56 (1981) 53-61 North-Holland Pubiishing Compxty
RESONANCE ENHANCED LASER IONISATION MASS SPECTROMETRY OF FOUR AROMATIC
MOLECULES
Charles T. RETTNER and John H. BROPHY ** MassachusettsInstifuteof Technology, Cambridge, &fa.szuchucetts0.2139. USA l
Received 12 September
1980
Multiphoton idnisation and fraflentation of aniline, benzene, N,Nilimethyl and 2,4dimethyl aniiine has been studied by laser ionisation mass spectrometry under collision free conditions. All four molecules undergo efticient resonanceenhanced w-photon ionisation (R’LPI) with relatively low laser intensities at h = 266 run producing the parent ion almost exclusively. At higher intensities, higher order processes compete producinp extensive fmgmentation. At 266 nm, all the major fragment ions are produced by R3Pl. For aniline excited at 294 run, energetic considerations suggest R4PI formation of fragments with differences in fragmentation between 266 and 294 nm reflecting the differing orders and energies above threshold of the competing processes. Comparison of R2PI efficiencies in aniline and benzene shows that the cross sections for ionisation of the resonant intermediate lBz excited state iF. both molecules are approximately quaI and independent of wavelength in the range 250-300 nm.
1. Introduction The very high proton fluxes generated by lasers have greatly facilitated the study of multiple and multiphoton processes. Initial experiments in the field of multiphoton ionisation (MPI) were concerned with the physics of the phenomenon [ i,Z] but the increasing availability of high peak power lasers has been accompanied by a rapid growth and divergence of the MPI literature (for recent reviews see ref. [3]). Aiongside a continuing interest in the basic MPI process, various workers have stressed spectroscopic [4,5], mass spectrometric [6-81, analytical [9-l 11 and diagnostic [12,13] applications of the technique_ Many molecules have been studied, notably NO, I3 and benzene, and a variety of conditions employed. Most studies have been made on bulk gases, but molecular beams [6,12,13] artd even liquids [ 141 have also beeti employed; Detectors range from s$npIe,
* To-whom all correw.n&nce rho&i be addressed, at :, Dep.aent of C&mistry, Stanford University, Stanford, California 94305, USA; ‘*&k&s&t address: Department of Cl&&try, lJ&&ity of Biiingham, Edgbxton, Birmii&k B15~2l%;‘U~IC.
.. 02.50 @No&-Hoiland ,, -. .. ..
. .. ‘.-0 301:-0I~/8I/~~-oooO/S .-. : _: : ..’
.:. _.-. ., .,
._
.
.
._.:
‘....
..:.
two-plate cells to mass spectrometers. When iorisation occurs via a real intermediate excited state, reached by a coherent m-photon excita-. tion, the process is resonance-enhanced (REMPI). The intermediate state then absorbs n further protons to the continuum, ionising the molecule. For m = n = 1, resonance-enhanced two-photon ionization (WI) results and very efficient ionisation may be obtained with comparatively weak lasers. In a recent study [15] it was shown that aniline readily undergoes EUPI when pumped in the region of its 1Bs -‘A, transition at wavelengths between 2!30300 nm. Experiments in molecular beams and bulk gas samples showed the process to be highly efficient and the technique of lU.PI was demonstrated as a
means of monitoring trace amounts of aniline in atmospheric air [9]. Other workers have since confumed these fmdings. Dietz et al. [ 131 have recently reported very similar results using a jet-cooled aniline sample, and Frueholz et al: [i l] have applied the R2PI technique to the detection of naphthaIene vapour, arriving at a similar estimate for ultimate detection limits. In the previous experiments with aniline, the relatively 1oWlaser i&&ties limited the overall order Publiig
ampay
..
54
CT. Rettner, J.H. Btvphy j&fuss spectrometry
of the ionisation process and mass analysis of the ionic products showed almost exclusive formation of the parent ion by R2PI. At higher laser intensi:ies, however, higher order processes may compete with R2Pl and other relaxation rates to produce fragment ions. Here we report further details of the laser ionisation and fragmentation of aniline together with results for two isomeric dim&y1 anilines, N,Ndimethyl and 2,4dimethyl aniline. We also report preliminary results for benzene, complementing other similar studies of this molecule at different wavelengths [6,16,17].
’
the Iascr beam power, the sample being introduced instead as a bulk gas via a needle valve. Also, two different lasers were employed. All experiments involved the use of a Quanta-Ray (DCR) Nd:YAG laser;equipped for the generation and separation of the second (532 nm), third (355 nm) and fourth (266 nm) harmonics. In a series of experiments with aniline, the second harmonic was used to pump a Molectron Rhodamine 6G dye Iaser. On doubling with a KDP crystal, this gave wavelengths tunable in the 290-300 run range with a5 kW peak power at a repetition rate of 10 Hz. Several experiments were carried out with the dye laser, but most studies employed the Nd:YAG fourth harmonic directly. This was obtained with an ~15 ns pulse duration at pulse energies in the range 0.03-25 mJ, again at 10 Hz. Over this range of pulse energies, the pulse duration was observed to be invariant, as was the mode structure of the output beam. Hence average power measurements, taken on a Scientech 362 power meter, were used to estimate peak laser powers.
2. Experimental A schematic of the apparatus is given in fig. 1.
This differs somewhat from that employed previously [IS]. Most notably, the moIecular beam source chamber was removed to allow direct monitoring
of aromatic molecules
of
7
au40
IRISES
I
,.
”
/
Fig. 1. Scheniatic of the laser ionj&t
..
-. ._
.-
._....
/-
.,._ ,_ _ _;. ::..,, _.%_ . .. ‘. :.. . ... .: ..,,. ,._ :._ ‘. .-.:_ _:_ ;.~. ,!I__ ,. ,.._‘‘.. .,. .’ .. :i. .- : I:.: ..__,.._,:, ;:I...._; .. :. : ,.:.. : ,,I,._ ,. ” ‘: ,_ ‘. . . . .., : ,,: ._-_.;. :. ._ _,-.-_.; ..i : ;._;:’ :.:..-. ,;;: .:, _ ..,._.. : . .I .‘.‘.:‘: :: ‘_., ;’ ,._.’_ ;_: :., :_ : .1 .. -,, ,. ._.‘: .,--
.. - :,_,,’ ,,
:.
‘: ,.- ..
Nd:YAG
.1. : .:,
.. ., _I
CT. Retmer,
J.H. Bmphy
/Mass spectmmerry of aromatic moiecdes
To check the linearity of the power meter and to observe the laser pulse duration, a portion of the beam was split off with a quartz plate for detection on an RCA lP28 photomultiplier. Two quartz lenses of 5 cm focal length, held on XYZ translators with a separation of approximately 10 cm, served to “collimate” the laser beam and also to provide fiie adjustment of the beam direction. A retractable 30 cm focal length quartz lens was employed in some experiments to bring the beam to a focus at the ion source. Two irises secured to the frame of the mass spectrometer housing served 2s an aid to alignment and to further collimate the beam. The beam was passed via a quartz window through the ionisation region of an EAI 250 quadrupole mass spectrometer. This instrument allows a clean passage through the ionisation region without modification. With the collimating irises fully open the (low-power) beam couId be imaged after passing through this region and positioned so as to be midway between any shadow-casting components of the ion source.
100
1a.’
’
’
’
’
+100
ni
55
On closing the irises, simple line-of-sight observation revealed a clear passage through the Centre of the ionisation region. This alignment is important, since photoe!ectrons are produced when high-power lasers strike metal surfaces, producing electron impact ionisation of the sample. The wavelength sensitivity and absence of air peaks in the mass spectra provide further evidence that this was not occurring. Signals were taken directly from the mass spectrometer electron inultiplier without preamplification and fed either to a Keithly electrometer or to a boxcar signal averager and, in both cases, displayed on a chart recorder. Maximum time-averaged output currents corresponded to u10-8 A. The samples (reagent grade) were used without further distillation, but were subjected to three freeze-thaw outgassing cycles before use and their purity checked with conventional electron impact mass spectrometry. The base pressure in the apparatus was heId at S.5 X lo-* Torr by a S-inch diffusion pump baffled with a liquid nitrogen cold trap. Maximum sample pressures were ==10M5 Torr. Although somewhat high for ion work, other workers have found 1231 ion coilision effects to be negligible under similar conditions.
3. Results
3.1. Atlilirze At moderate laser intensities in the region of the O”,band of the ‘B2--‘AI transition of aniline (293.8 run) very soft but efficient ionisation is achieved. The laser ion&&ion mass spectrum consists of a single parent ion peak at M/e = 93 [ 13,151. As the laser intensity is increased to 20.1 hlW cmw2,however,
IO 208040808076’8090
We. Fig. 2. hIzss.spectra
&the ion fragments produced by laser ionistion of aniliie’excited via’its.‘Bzl’Al transitioa’(a) Excitation of the 0: band at 293.8 nm with a laser intensity -==OS.hfW&II-~; @) excitation at 266 Mlwith B k&r intensity ~0.2 MWcm-2:~(cj’cx~tationst.266.nm With~l~r ix?te@ty’=l.GW cmS2. Notice’the z&l? char& in &gram (a) M/e:= 93 has been divided by 100; and’in (by&z =93 .. anhM/i =.66 have &eh divided by 1000 and’iO~respect&ely.; ..’ : : _‘. -:
., ... .
._
: ‘.
‘-
fragmentation becomes detectable. The laser ionisation (LI) mass spectrum of aniline at 293.8 nm is shown in fig. 2-a. Signal strengths permitted only crude mass resolution (2-3 maxs units), revealing peakscentred at Mje = 93,78,66,52,39 and 28. AS well as the p=ent ion [13,15], the intensity of the second strongest p&k (at M/e - 66) Was also found
to follow the on&photqn absorption spectrum. No measurements w&k mid! on the qther pe+s. The : fi&nentation pafi&ti found to be relatively ‘insensitive to~sn@l wavelength changes r for example, excitation at 294.2 nm produced a sir@ar mass spec-
CT. Retmer. J.H. Bmphy j
56
Massspectmn;etiy o:ammntic molecules
trum. In order to test the effect of a huge wavelength change, the fourth harmonic of the Nd:YAG laser was used directly. This beam, largely due to its superior fluence, was found to be much more suitable for detailed study of the power dependences of the fragmentation pattern. LI mass spectra for excitation at 266 nm employing two extreme beam intensities are given in figs. 2b and 2c, respectively. At the highest intensity, fig. 2c, in addition to the major mass peaks listed above, fragment ions appear at M/e=12and 14. Fig. 3 displays the power dependences of some of the more prominent peaks, and their limiting intensity dependence-s are given in table 1. Although power dependences were taken with the same (low) resolution, at higher laser powers signals were sufticiently strong to allow unit maSS resolution_ In fig. 4 a detail of the mass scan is shown forfif/e = 73-80. For comparison the 70 eV electron impact ma= spectrum recorded in the same sample is indicated in the figure.
=-Parent
Table 1 Limiting intensity dependences
log (signd) a) A log (ktensity)
MolecuIe
A
aniline
93 78 66 52 39 28
2.0 2.4 3.0 3.0 3.25 3.1
f 0.05 2 0.2 b 0.1 f 0.1 c 0.1 + 0.1
2,4dimethyl aniline
121 106 77 42
2.0 3.0 2.9 2.9
i 0.05 f 0.1 t 0.1 L 0.1
N,NdimethyI aniline
121 105 79 41
2.05 2.8 3.2 2.8
+ 0.05 = 0.2 f 0.2 t 0.2
benzene
78
1.95 ? 0.1
a) Obtained from least squares analysis of data for a range of low laser intensities yieIding linear plots.
Ion
A-Mass 66 +-Mass 39 W-Mass 28 ~-Mass I
78
I
lci’
#O 10
IO2 103
Laser. lntensuy/MW cme2 . Fig 3. &rent and fmgment ion yield curves from the laser: : io&tiod of aniline at.266 nm as a’funckon 0: the Ia&. ‘.inter&. 15,e line& plots at I&v interisitiiwer~ dbtained _:~ .,,u!itha_coiIitqatedbeam (see t&t), whks tie c&es at .: -high pow& correspond to a focused beam. Only.iepreseti-’ ; .. M&q data points and enor bars are dig&d foi clarity_ :
__-
.
:
I
I 73
-1. ..
I I 74.75
I 78-n
.. .,:: -.hl+.
I
I I 7a.R’8Q.
I
~. :‘. :;_._
..
F&P. &it mass rklution’df the &up of.ionit’&nents belsknM/e 2 73,and 80 prdduced in the lker~ioniktion of anilineat A A.266 rq_,Fbr cqmp+n the electron impact ~, masz qkctrum re+ded in the same s+pIe,~k’&&c;ited_b~’ ‘.
CT. Rettner. J.H. Brophy /Mau spectrometry of aromatic molecules
No ionisation could be obtained with the green 532 nm second hamkmic up to the maximum intensities available (=I00 GW cm-*). This is in keeping with the findings of Dietz et al. [ 131. A similar negative result was obtained with third harmonic at 35.5 nm. 20
3.2. 2,4- and N,Ndimerhyl aniline
30
40
50
60
70
80
”
We
Ft. 6. Laser ionisation rna~ spectra for benzene for a laser Laser ionisation signals from these two compounds were found to be vexy similar in magnitude to those of aniline. Fig. 5 shows LI mass spectra for these two compounds at 2-3 mass unit resolution, taken with the fourth harmonic of the Nd:YAG at 266 nm. Power dependences for the major peaks over a range of low laser intensities are given in table 1. Again, the second and third harmonics failed to produce ionisation.
intensity ==l WV cm-* at 266 nm. Note the scale change for the parent ion peak, which is divided by 100.
-120. This large reduction in signal made estimation of the limiting low power intensity dependences impractical for all but the parent ion, which gave a slope of 1.95 + 0.1, confirming the ionisation as a two-photon process.
3.3. Benzene
4. DiscuSion
An LI mass spectrum, again taken with 2-3 mass unit resolution using the fourth harmonic of the Nd: YAC, is shown in fig. 6. Electron impact and LI mass spectra were taken for an aniline-benzene mixture, from which it was found that at 266 nm the R2PI process is more efficient for aniline by a factor of
4.1. Aniline The highly efficient laser ionisation of aniline when pumped in the region of its ’ BZ-‘A, transition is again demonstrated. It may be concluded that the second photon excites the aniline to an autoionising state which is comparatively slow to predissociate into neutral species [ 131. Two photons at 294 nm correspond to 816 kJ mol-’ compared to the ionisa-
z 1100
s~laC&k&HCN
3looO
-Fig. 5. +ser ion&ion III= _$~eCtrsfoi (a) 2,4dimethyl an~~~e-~d~@).N~Ndimethyl anilinp for a laser intensity. zO.2 hW~cm~*,ati66~nm. The regive inteNity bf the +A aiM/t+ Y 93 ir;(& @&I from e+erimckt tb expe&ent z&d: is att&&l to a&& &pu;ity.‘Note Uk. kale d&&s ftir .‘. theparent ionpeaks;~ksea~dividedby.IOO, .‘I .. :,- :
I
,
Fig. 7. Schematic representation of the energetics of ionintion md fragmentation of aniline.Theheats of formation of .repRsentative pr.&ucts xe shown fx comparison with_the &erg) corresponding t+.bsbrption of 23nd 3 photons df -. laser~t~t266and294nm.
58
CT. Retmer. J.H. Brophy /Man spectrometry of aromatic nwtecules
tion potential of aniline, which is 740 k3 mol-’ [I 81. The remaining energy is insufficient to fragment the parent ion, which explains the “soft” nature of the ionisation at low laser intensities. At higher intensities, the probability for absorbing further photons becomes significant. The energy level diagram shown in fig. 7 reveals that even three photons at 294 nm may not be enough for significant fragmentation, particuIarly since there are almost certainly activation barriers for these rearrangement-dissociation processes. Fragmentation to the cyclopentadienyl ion plus HCN would be possible, but the more energetic linear &Hz ion was found to be favoured in a conventional eiectrcn impact mass spectrometric study [19] of the decomposition of C6H5NH:. The fact that the ionisation efficiency of the “C:” mass 66, peak also followed the one-photon absorption spectrum stiggests that the same one-photon resonance enhancement applies to the generation of the fragment ions. Thus we conclude that at least four pho-
tons are required to produce the fragment ions shown in fig. 2a, i.e., m = I, n B 3. Whereas excitation at 293.8 nm was known to pump the 0: band of the ‘BZ-‘AI transition, the exact transition pumped by the 266 nm beam is not known. However, the resonant state is almost certainly the same IBz state [20], but with considerably more vibrational energy:. AIthougb a small wavelength change to 294.2 nm did not produce a significantly different fragmentation pattern, the larger change to 266 nm had a substantial, but not drastic effect. Comparison of figs. 2a and 2b requires some caution, since the two spectra were not recorded with identical laser beam pro?des or power. The gross differences observedare however attributed to the wavelength change. Two such differences arc apparent: (i) the degree of fragmentation is greater at the shorter wavelength, and (ii) the mass 66 peak represents a much larger fraction of the fragment ions at this wavelength_ We will return to this point later, after considering the meaning of the power dependences observed for excitation at 266 run, fig. 3. Multiple and mtitiphoton ionisation processes [ZIP can be described by a rate equation approach ]3,17, 22,23]_ By model& the process as a many-!evel sysi tern, exprtions for the various ion populations may be obtained. Ln a previous study [15].we used such an approach to obtain an estimate for the photoionisa-
tion absorption cross section for the *Ba state and found it to be =3.5 times larger than that for the initial ‘BZ-‘A, transition_ Others have recently attempted to ratio&se ion yield curves with this approach [ 17,231. In the most comprehensive treatment to date, Robin and co-workers [23] derive expressions for the populations of fragment ions under a variety of conditions. What emerges from these treatments is that in the limit of low-intensity fragment ion yields should increase as Imen> where the ion is produced following the absorption of m +rt photons. In most cases signals do not persist to this limit. The suitability of aniline for this type of study is again emphasised. The Linear slopes exhibited at !ow laser intensities, fig. 3, are taken as indicative of the number of photons required to yield the respective fragment ions. It is seen that, apart from the parent ion and the peak at mass 77, the other major fragments exhibit cubic intensity dependences consistent with formation via a three-photon process. The dependence of the mass 77 peak was found to be non-integer even at the lowest intensities reached. In the absence of an alternative explanation, we attribute this anomaly to an impurity. For example, a trace quantity of benzene would give a twophoton dependence to be added to a three-photon contribution from a “‘genuine” aniline peak. The very high efficiency of R2FI processes makes such interference a real problem when looking at higher-order frngment ion peaks. The gross differences in the laser ionisation mass spectra for exc.itation at 293.8 nm and 266 nm may now be rationalised. We believe that excitation at both wavelengths leads to two-photon formation of the parent ion. At 266 nm fragmentation occurs to yield other ions, predominantly via an overall threephoton process. These same fragments require four photons upon excitation at 293.8 nm. We believe that this difference in order for the process accounts for the reduced fragmentation at the longer wavelength. The differing relative importance of the mass 66 fragment may also be rational&d in terms ,of the differing excergicities of the various fragmentation &annels. We attribute the has 66 peeto the.linear &Hz ion. This species, together with its companion .. product, HCN,.represents the-lowest ener& of +rry of the fragmentation products and most likely results
diskxiation path. In the.case ‘.
from theldwest,eneqg
:, _’ ..;
‘. _‘.
....
.. I’_1 .,
:
C.T. Retuner, J.H. Brophy /&iass spectrometry
of three-photon fragmentation at 266 MI, the small ener,v differences between the various dissociation paths will be important since the three-photon energy exceeds the fragmentation threshold by only ~150 kJ mol-r . This leads to predominant formation of the lowest energy products, specifically the &Hz ion plus neutral HCN. Four-photon excitation at 293.8 nm, however, exceeds threshold by -30 W mol-’ , and the various fragmentation rates become more nearly equal. The other major fragment ions are (tentatively) assigned to the formulae ChH; (mass 77); C4H$ (52); CaH; (39); and H,CN’ (28), all of which are major peaks in the electron impact spectrum of aniline. As the laser intensity is increased, the integer dependences of the ion yields on laser intensity are lost as saturation effects, ion photofragmentation and multi-path processes become important. This leads to the appearance of new peaks in the mass spectrum, e.g. at M/e = 12 and 14 in fig. 2c, and to a gradual equahsation of peak heights. The same general trends have been reported by others [6,17,23,24], some of whom have attempted a fit to the rate expressions mentioned above. We do not feel that such a treatment would be fruitful here, due to the complexity of the system. Specifically, there are many unknowns, such as radiative rates, adsorption cross sections, the locations of sink states, etc. Furthermore, the exact geometrical factors pertinent to our system as saturation is approached with a focused beam are, as yet, undetermined. 4.2. 2,4-and
N,Ndimethyl
aniline
In a study of the molecules naphthalene and azulene, Lubman et al. [24] demonstrated that LI mass spectrometry can readily distinguish isomers of large organic molecules. The spectra shown in fig. 5 for the two diiethyl anilines considered here support
this finding. Although not explored here, the R2PI spectra of the two molecules would also show differences imposed by their differing one-photon absorption spectra. As su&, the LI mass spectrometry technique offers an extra dimension to conventional electron irnpact,mass spectiometry. In fairness, the fragmentation patte@ observed here show no more signitkant-differences than the converitional mass spectra 1251. ‘khe fact that ions are obt&ed at 266 . :’
of arornbc mokcules
59
run and not at 355 run or 532 run does give further information, indicative of the chemical and spectra-’ scopic sirniiarities of the isomers. The ionisation potentials for the 2,4 and N,Ndimethyl aniline are 714 kJ mol-’ and 685 kJ mol-’ , respectively, compared to that of 740 kJ mol-’ for aniline [ 181. Hence, the energetics of the various processes are essentially identical to that described for aniline. The power dependences given in table 1 for the major fragment ions of both isomers indicate three-photon production of these species. For 2,4 dimethyl aniline, these are assigned as [CHs - &HsNHs]+ (106); CsH: (77); and CHaCHN* (42). Likewise, for N,Ndiiethyl aniline
we have CeHs.NCH:
(105); CeH: (79); and
CHsCN’ (41). As with aniline, the fragmentation is qualitatively similar to that found in electron impact mass spectra [25]. Whereas the parent aniline ion readily loses HCN, both dimethyl aniline parent ions display a propensity far loss of a methyl group [26]. 4.3. Benzene As a prototype molecule, the MPI of benzene has been (comparatively) well-studied, beginning with the pioneering work of Johnson [27] and more recently taking the form of LI mass spectrometric studies. Such studies have been reported by three groups. Zandee and Bernstein [6,28] investigated the fragmentation of benzene in a molecular beam using a near-W dye laser excitation source focused to intensities of up to 2 X 10” W cm-‘. Since they were exploiting an initial two-photon resonance, such high powers were necessary to give adequate signals but resulted in considerable fragmentation. Boesl et al. [ 161 demonstrated R2PI of benzene in a molecular beam using much smaller laser intensities (~1 MW cm-*). Consequently, they observed only the parent ion (cf. refs. [13,153 for aniline)_ Most recently, Reilly and Kompa [17] employed KrF (248.5 nm) and ArF (193 nm) laser lines with pulse
energies of up to 9 ml, using a 10 cm focal length lens, from which we estimate intensities >1CKlMw cm-= . They observed increasing fragmentation with laser intensity that described The results other studies,
in a qualitatively similar fashion to above for aniline. reported here are consistent with these The fragmentation pattern shown in
60
CT. Rettner,. 3.H. Brvphy /Muss specnometry of aromatic nwleadPs’
fig. 6 displays Cs,C4, C5 and C6 peaks. Energetic arguments identical to those applied above, considering which species are energetically accessible as well as dynamically feasible, suggested the following assignments: CsH; (39); C4Ht (50); CsH; (63); and CsHz (78). AU of these are major fragments in the electron impact spectrum and were observed by Reilly and Kompa [ 173. Although it proved impractical to measure the limiting intensity dependences, energetic considerations [ 181 similar to those outlined for aniliie suggest that at least four photons would be necessary to produce the fragments observed_ This difference from the anilines is largely the result of the increased ionisation potential, which is =I50 kJ mol-* higher than for aniline, for example. Th facto: of W-120difference in the R2PI efficiencies for benzene and aniline was observed at low laser intensitiesand, therefore, approximates to the ratio olhua.4/u1n~au, where the subscripts 1 and 2 refer to absorption from the ground and resonant states, and A and B to aniline and benzene, respcctively. We estimate uIA (266 nm) to be slightly smaller than ul~ (293.8 nm) [IS], say 7 X lo-‘* cm’. Literature data-for benzene [29] indicates or8 = 5 X lO”e- cm*. Thus (JlAialB = 140, suggesting that @&,= f&B. This is consistent with previous observations if we assume that the wavelength dependence of these cios sections is slight. u;?B (248.5 nm) has been estimated as 3.4 X 10-l’ cm’ [ I7], which is close to the value of 3.5 X lo-” cm* that we reported for oaA (293.8 nm) [.I51_
5. Conclusions hiiine, benzene and the isomeric substituted &es, N,N.and 2,4&nethyl-aniline, have been. -shown to undergo efficient, resonanceenhanced; : multiphoton ionisation when excited via their ‘Et1_4I transitions in the W_,At EJ’ghla&r $te&itjr, extensive~frag&nti?& o&irk prodking & of..the. io&fiagmen*ts obs&ed ir&&e&onal electron I
As might be e?rpected for absorption http a largely structurelesscontinuum, the cross se&ions for onephoton ionisation of the resonant in&-mediate. ... excite-d states of benzene~aud aniline are approxi- s mately equal and independent of wavelengthabove. threshotd.
Acknowledgement We are grateful to J.L. Kinsey and R-W. Field who made facilities available in the spectroscopic.laboratory at Massachusetts Lnstitute of Technology; We also acknowledge partial support of this work by theNational Science Fou&ation and the US Office of Naval Research.
References [l] P.D. Maker, R.W. Tcrhune and C.$t. Savage, Quantum EIectronics,(Columbia Univ. Press, New York, !964) p. 1559. [ 21F.V. Bu&in and AM. Prokoro;, Sov. Phys. JETP 19 (1964) 739;. L.V. Keldish,‘Zh. l&p. Tear. Fti, 4.7 (1964) 1945. [ 31 D.H. Parke~:J.O. Berg and && El-Sayed, in: Advances. in Laser Chetiistj, ed. A.H,Zewaii (Springer, B&in, 1978); P.M. Johnson, Accounts&em. Res.13 (1980) 20. [4] G.C. Niemvl and S.D. cois011, J. Chem. Phys. $8 (1978) 5656. [5] K. Krpgh-Jespersen, I&P. Rava.and L Good&i, Chem. Phyr44 (1979) 295. [6] L Zandy and R.B:Be&eiq J. &m. Phyr 70 (1979) 2514.
:
61 -i 15j J.H:‘&ophy and CT. Rettuer, &em. f1979)-351.
Phys. Letters 67 ’
[is] U.Boesl,H.J.NeuserandE.k.Schiag, z.
Naturforsch.
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