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Infrared photofragmentation spectra of C2H4·(NO)n+ cluster ions
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 1.59(1996) 185-196
Infrared photofragmentation spectra of C2H4.(NO)L cluster ions A. Mouhandes, A.J. State” School
of Molecular
Sciences,
University
of Sussex,
Falmer,
Brighton,
BNI
9QJ, UK
Received 19 April 1996; accepted 10 June 1996
Abstract A line-tuneable CO2 laser has been used to excite one or more vibrational modes in ethylene molecules resident on sizeselected C2HJ(NO)i cluster ions, for n in the range 2-20. Excitation is followed by the observation of three separate fragmentation channels: -C2H4, -NO and -2N0, whose relative intensities are found to be extremely sensitive to cluster size. The most marked transitions in fragmentation pattern are found to occur between odd- and even-sized cluster ions, with decay channels favouring the formation of even electron ions. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles. Those profiles associated with several of the even-sized ions show evidence of two prominent absorption features, one of which is attributed to the v, infrared active vibrational mode in ethylene. A second absorption feature is tentatively assigned as the yg mode which, under normal circumstances, is Raman active; however, a mechanism is proposed whereby the ethylene molecule could become distorted. Keywords:
Fragmentation
channels; Infrared absorption profiles
1. Introduction In a recent series of experiments, we have demonstrated that, through a careful selection of cluster-chromophore combinations, it is possible to study the spectroscopy and decay dynamics of single neutral infrared chromophores in association with mass-selected cluster ions [l-4]. The advantage offered by these experiments is that it then becomes possible to correlate the evolution of spectral features with other experimental observables, such as mass spectra and the decay patterns of photoexcited cluster ions. Thus, changes in absorption frequency can be equated with the presence of ‘magic numbers’ and/or the introduction of new * Corresponding author.
fragmentation routes as identified in a mass spectrometer. The key aspect of these experiments is that, in heterogeneous clusters of the form XY,, the chromophore X should have an ionisation energy (IE) which is larger than that of the host cluster Y,. Under these circumstances, the cluster can be ionised in the knowledge that it is Y, which carries the positive charge [l-4]. Only under very unusual circumstance do we observe infrared-induced charge transfer between the chromophore and the host cluster [5]. A disadvantage of the approach discussed here is that there is only limited control over the internal energies of the cluster ions. Electron impact ionisation is a comparatively violent process and capable of creating a broad range of excited states; however, because observation time scales are of the order of 10-5-10-4 s, many of these
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states decay to leave a residual vibrational energy in the ground electronic state. This energy can then promote the break-up of cluster ions during the course of an experiment. Since unimolecular decay is still observed after 10e4 s [6], it is known that a certain fraction of cluster ions possess an internal energy which is, at least, equivalent to the minimum dissociation energy, Ei. However, because gas cluster ions of the type discussed here are, for the most part, weakly-bound, their internal energies cannot be very much higher than Ei, otherwise extensive fragmentation would occur prior to reaching the point of laser excitation. Hence, the energy deposited from an IR photon (-0.11 eV) is a significant fraction of the total internal energy and, as a consequence, photofragmentation signals are almost always found to be very much stronger than those recorded for metastable decay [l-4]. An estimate of average internal energy can be obtained from the measurement of kinetic energy releases associated with unimolecular decay [7,8].
Magnet Laser Port
,.
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Reported here are the results of a series of measurements undertaken on the IR photodissociation spectroscopy of C,H,*(NO)i clusters for n in the range 2-20. From a summation of fragment ion intensities it has been possible to derive vibrational absorption spectra as a function of cluster size, and these are used to discuss the structures of the ions. Both the fragmentation patterns and the recorded absorption profiles exhibit features not seen in related experiments on other cluster ion systems.
2. Experimental
section
Mixed neutral clusters of the form C2H4*(NO)n were prepared using the “pick up” technique on an apparatus that consists of a supersonic nozzle coupled to a modified double-focusing “reverse geometry” VG ZAB-E mass spectrometer. Fig. 1 shows a schematic diagram of the apparatus and details of the exact construction have been given
i Magncttc
159
i- .
ESA Laser Port
:. ..
Vacuum Generator ZAB-E, reverse geometry double focusing mass spectrometer hzieE+
c :,
6
” .c”
Nozzle CO2 Laser
Ion Source
Cluster Chamber
Fig. 1. Schematic
diagram
of the apparatus
A. Mouhandes,
A.J. Stace/Intemational
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of Mass
previously [6]; the only additional feature being a line-tuneable CO* infrared laser (Edinburgh Instruments PL4). Neutral clusters of NO were generated via the adiabatic expansion of a mixture consisting of 1% nitric oxide in argon through a 200 pm diameter pulsed conical nozzle. Following collimation through a 1 mm diameter skimmer, the cluster beam entered a flight tube approximately 70 cm long, in which the background pressure had been increased to -lo-’ mbar through the introduction of C2H4 via a needle valve. During their passage through this region, some of the NO clusters formed collision complexes with C2H4 molecules and optimum ion signals were observed when the nozzle stagnation pressure was approximately 60 psi. Ionisation to C2H4.(NO)n+ was achieved using 70 eV electrons, and use of the “pick-up” procedure [l-4] proved very successful in generating stable ion currents with almost uniform intensity for clusters consisting of a single chromophore and up to twenty host molecules. One disadvantage of using an nitric oxide-argon mixture is the high probability of forming mixed clusters. Although the masses of clusters can, for the most part, be identified separately, there is the problem of a mass equivalence between four NO molecules and three Ar atoms (120 amu); thus large C2H4(NO)n+ clusters could contain some argon atoms and still have the same nominal mass. Experiments showed that, following photoexcitation, the mixed clusters gave preferential loss of argon rather than NO. Hence, the decay patterns of pure C,H,(NO),’ clusters could be monitored accordingly; however, parent ion intensities could not be used to normalise the data, instead metastable (unimolecular) peak intensities (see below) were used for this purpose. Experiments with a range of vapour-gasargon mixtures [2,3,9] suggest that the presence of argon atoms in the clusters immediately following adiabatic expansion, plays an important part both in the C2H4 attachment process and in stabilising the ion complex following ionisation.
Spectrometry
and
The initial the form: (NO);Ar,
-
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“pick-up”
collision
+ C2H, -
[C&W&J&,]
C2H,*(NO),
185-196
probably
187
takes
+ mAr
where [C2H,(NO),;Ar,]# denotes a collision complex which is ultimately stabilised through the evaporation of the argon atoms and, possibly, some of the nitric oxide molecules. The argon atoms would depart first since, of the species present, they should have the lowest binding energy; a similar stabilisation step probably accompanies ionisation. Attempts to perform “pick-up” experiments in the absence of argon, for example, using expansions of pure NO or COZ, have always resulted in failure [2,3]. Ions were extracted from the source with a potential of 7 kV and size-selected using a magnetic sector. These ions then entered a field-free region where they were irradiated with photons from a line-tuneable carbon dioxide laser (frequency range 920-l 100 cm-‘, resolution -1.5 cm-‘), which was aligned coaxially with the ion flight path; the overlap between the laser and ion beams covered a distance of approximately 150 cm. The masses of any ionic photofragments were determined using the electrostatic analyser in the MIKES (mass-analysed ion kinetic energy spectra) mode [lo]. To eliminate possible interference from fragment ions due either to metastable decay (see below) or collision-induced dissociation (CID), the laser beam was modulated at half the nozzle frequency and foreground-background subtraction performed on photofragment signals. Two data collection techniques were employed: photodissociation and unimolecular decay signals at a.single kinetic energy were recorded using gated photon counting via a scintillation (Daly) ion detector; mass spectra were recorded using phase-sensitive detection in conjunction with analogue output from the ion detection system. To keep CID processes to a minimum, the background pressure in the flight tube was
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Metastable decay pattern of C,H4(NO)n’ loss of NO I ’ * ”
1”
n ’ u ’ n ’ n ’ u ’ ”
1
1.0
/
0.0 I,
1
2
4
t
I,
I.I
6
8
I
I,
I.
IO
12
II1I.I I
I4
I,
I
16
18
‘1
I
20
n in C,H,(NO),+
(a)
Metastable decay pattern of C,H,(NO),C
0.8
loss
of 2N0
n
h
.3
0.0
(4
I, 4
I1 6
10 8
I1 10
I. 12
I 14
I
I 16
I
I1 18
I 20
n in C,H,(NO),+
Fig. 2. Observed metastable fragmentation patterns for C2H4(NO)i cluster ions plotted as a function been plotted separately in order to emphasise the odd-even alternation in signal intensity.
of n. The two fragmentation
channels have
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maintained at 6 x 10e9 mbar. Photodissociation signals were between lo4 and 10” counts min-’ against a background signal of
3. Results and discussion 3.1. Metastable
decay pattern
Following mass selection, three separate unimolecular (metastable) decay routes were identified for C2H4(NO)i cluster ions: C2H4.(NO);
-
C2H, + (NO);
C,H,.(NO);
-
C,H,.(NO);-
(1) i + NO
n>3 (2)
C2H4*(NO)n+ -
CzH4.(NO)&
+ 2N0
n>3
(3) Note the very specific nature of reaction (1); this decay route is not exhibited by any other size of cluster. In an earlier study of SF6(NO),’ the unimolecular loss of SF6 was also found to be size-specific, but not to the degree seen here [3]. Fig. 2 shows the relative intensities of
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189
fragment ions from the two metastable decay routes (2) and (3) above, plotted as a function of cluster size. The marked alternation in behaviour between odd- and even-sized clusters shown for C2H4*(NO)i is quite pronounced and follows a pattern that is mirrored by the parent ion intensities. The fragmentation patterns recorded for SF,(NO)i clusters also exhibited alternations of the type shown in Fig. 2 and, the fact that similar patterns of intensity fluctuation are also seen in the mass spectra of both (NO): and SF,(NO)’ clusters [3,11-131, suggests that the phenomenon is determined by the stability of the underlying (NO); moiety. The NO molecule is unusual in that the ground-state electronic configuration
-tm
(~2~)~
(~~J2
@2$
(nJ4
G.pl
gives it a single unpaired electron in a a antibonding HOMO. In contrast, NO’ is an evenelectron ion (‘C’) with all the valence electrons present as pairs. We believe this characteristic extends to (NO);, and that the particular stability of this closed-shell ion accounts for reaction (1) and also results in (NO); acting as a central core for larger positively charged ions. Such a core would have to be an odd-sized ion in order to explain the decay pattern observed for the larger clusters in Fig. 2 (see below). These assumptions regarding (NO); are supported by separate CO (carbon monoxide) laser photofragmentation studies of SF,(NO),’ and (NO),’ clusters, which both show strong evidence of (NO),’ being a stable infrared chromophore w41. If the above picture is now extended to the larger clusters, then it can be seen that all oddsized C2H4(NO)i clusters will be even electron systems; these are the most intense ions and they prefer to lose 2 NO molecules in order to maintain the stability associated with n = odd. In contrast, even-sized C2H4(NO),’ are odd electron systems; these are at least an order of magnitude less intense than n = odd, and they exhibit a strong preference for the loss of just one NO molecule in order to generate stable evenelectron clusters.
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Photoinduced loss of NO for C,H,(NO),’
0.8
2
4
6
8
IO
12
14
16
18
20
n in C,H,O\IO),+
(4
Photoinduced loss of 2N0 for C,H,(NO),+
0.8
0.2
0.0
(b) Fig. 3. Observed photoinduced fragmentation mately 967 cm-’ and the two fragmentation intensity.
n in C,H,(NO),’ patterns for C2H4(NO).’ cluster ions plotted as a function of n. The photon energy is approxichannels have been plotted separately in order to emphasise the odd-even alternation in signal
A. Mouhandes,
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3.2. Infrared photofragmentation profiles
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of Mass Spectrometry
patterns and
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191
small fraction of the photon’s energy is dissipated into the cluster ion. In addition to the IR active mode, ethylene has a second mode, vg, at 940 cm-‘, which is Raman active. There has been a suggestion that following the formation of a neutral ethylene dimer, the latter mode may acquire infrared activity as a consequence of a v7-vg Fermi resonance [18,20]. In the case of C2H4*(NO)n+ cluster ions, the magnitudes of any shifts and the degree of interaction between modes may well be accentuated by virtue of the presence of a positive charge; evidence to this effect is to be found in a recent study of SF,.Ari cluster ions [4]. Fig. 3 shows the relative intensities of the photo-induced signals recorded for reactions (2) and (3). As might be expected, the fragmentation pattern follows that given for metastable or unimolecular decay; the reason being that a single infrared photon does not add a significant amount of extra energy to an ion. In effect, the factor(s)
Within the range of the CO1 laser, CzHJ has one infrared active mode which is v7 (Br,) at 948.8 cm-‘. In association with a broad range of atoms and molecules, the infrared predissociation of neutral ethylene dimer complexes has been the subject of very extensive study [1518]. In the experiments discussed here, the photo-induced decay mechanism is considered to be very similar to that identified in earlier work: C2H4 absorbs a photon and after some delay, a fraction of that energy dissipates into the cluster ion where it is used to promote dissociation. Recent studies of kinetic energy release following the decay of photoexcited SF&NO),+ [S] and SF6*Ari [19] cluster ions have shown that, following absorption of an infrared photon, the SF6 molecule undergoes state-to-state vibrational relaxation and only a C,H,(NO),+
loss of NO
1.2
1.0
.s 0.8 s f <
0.6
. 2 *z d 3
0.4
0.2
00 930
940
950
960
970
980
990
Photon Energy / cm-’ Fig. 4. Infrared absorption profile recorded wavelength. The points are the experimental latter are given as broken lines.
for C2H4.(N0)i and determined by summing all fragment ion intensities as a function of laser data and the solid line is a sum of the fitted Lorentzian profiles, where individual components to the
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Spectrometly
responsible for the pronounced fluctuations in fragment ion signal outweigh the perturbation imposed by the addition of a ~950 cm-’ photon. From a summation of photofragment ion intensities as a function laser wavelength, it has been possible to derive infrared absorption profiles, and examples of these are shown in Figs. 4-8. For each example the profiles have been fitted to Lorentzian line shapes where the parameters were position of the peak maximum and FWHM. In all cases, a least squares analysis of the residuals between experimental data and fitted profiles, gave an average error of Sloe2 on a scale where the data points are normalised to 1. Attempts to fit the data to Gaussian profiles always gave errors >>10e2. In a majority of cases, the profiles consist of two relatively intense peaks centred at ~957 cm-’ and ~966 cm-‘, respectively, together with minor contributions which appear at ~945 cm-’ and ~990 cm-‘, but do not follow any systematic pattern. Fig. 9 presents a summary of wavelength
I
loss of NO
I
1.0 1.0 -
~ .z c 7
159 (1996)
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variations seen for the two principal contributions to each profile, plotted as a function of cluster size. The plot shows two quite distinct absorption frequencies which are almost independent of cluster size and separated throughout the range by approximately 9 cm-‘. It is interesting to note that the frequency difference between v7 and vs in isolated ethylene is 8.8 cm-’ [21]. If the observed features do correspond to v7 and v8, then both are blue-shifted by approximately 15 cm-l with respect to the isolated molecule and, although such shifts appear quite frequently in these experiments [l-4], the above values are slightly larger than those seen previously. The widths of the two absorption features identified in Fig. 9 vary between 4 and 19 cm-’ which, although consistent with previous measurements on neutral ethylene complexes [15-181, show no obvious trends as a function of cluster size. One of the particular features of this experiment is that the cluster ions are mass- or
C,H,(NO),+ I
and Ion Processes
I
I
I
.
0.8 -
I
I
I
1
I
I
940
950
960
970
980
990
Photon Energy / cm-’ Fig. 5. As for Fig. Fig, 4, but for C2H,(NO);.
_
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of Mass Spectrometry
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C,H,(NO),,+ loss of NO
0.8
0.6
0.0 1
’
930
I 940
,
I
I
I
I
I
950
960
970
980
990
Photon Energy / cm-’ Fig. 6. As for Fig. 4, but for C2H,(NO);,,.
C,H,(NO),,+ loss of NO 12
10
I
I
I
I
I
I
-
0.2 -
0.0 930
’
I
I
I
I
1
I
940
950
960
970
980
990
Photon Energy / cm-’ Fig. 7. As for Fig. 4, but for C2HJ.(N0);2.
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193
194
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C,H,(NO), ,+ loss of 2(NO) I
I
I
1
A .’,‘.. ., .
0.8 vl .Z 5 e
0.6
2
.
.e $ z -
0.4
,.:.u ‘,.,
l
‘.
0.2
.’
,‘.
‘.
,’
.’
‘._.
_’
:.
_:
.:_
‘,
‘.
‘.
:
: .-.
:.:
...
.
._
:..i.Y;..:i
0.0
930
940
950
960
970
.
980
. .
990
Photon Energy / cm-l
Fig. 8. As for Fig. 4, but for C?H4$NO);,.
950 2
1,
I,
,,I,,
4
6
8
, 10
12
,
,
14
, 16
,
, 18
, 20
n in C,H,(NO),’ Fig. 9. Peak centres of the two principal
absorption
features
seen in each of the profiles,
plotted
as a function
of cluster size n.
A. Mouhandes,
AJ. Stacelintemational
Journal
of Mass Spectrometry
size-selected; hence we can be quite certain that the ions do not contain more than one C2H4 molecule. Therefore, the features seen in Figs. 4-8 cannot be equated with two molecules on the same cluster occupying different sites. The only obvious possibilities are: (i) two distinct types of C2H4*(NO)i cluster are being generated for each value of 12and, in each case, the resident CZHa molecule is located in one of two possible positions that are sufficiently different as to cause a relative shift of 9 cm-’ in the v7 mode; (ii) more than one IR active mode falls within the wavelength range scanned by the laser. An interesting relationship is to be found if the ratio of the integrated intensities for the two absorption features shown in Fig. 9, is plotted as a function of cluster size. That result is given in Fig. 10, where it can be seen that there is a marked variation with cluster size, with the component at lower frequency (possibly vs) being the more intense on even-sized cluster ions. As discussed earlier, even-sized (NO),’ ions will contain an odd number of electrons and,
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195
because evidence from condensed phase experiments supports the formation of stable NO dimers [22-251, large even-sized cluster ions might be expected to contain, on average, a single unpaired NO molecule and, as a consequence, an unpaired electron. Under those circumstance, any interaction between the latter and the ethylene molecule is most likely to proceed via the r antibonding LUMO orbital on C2H4. From UV electronic spectra (an extreme case) [26], it can be seen that any introduction of electron population into the b, antibonding orbital on ethylene results in considerable distortion to the molecule. Thus, any interaction between a single NO and C2H4 can be expected to distort the latter, at which point the vs vibrational mode should become infrared active. From Fig. 10, it can be seen that what we assume to be v8 acquires considerable intensity in the even-sized cluster ions which, for a cluster like (NO);,,, suggests a configuration of the form: (NO)$[(NO)&NO, where the single nitric oxide molecule contributes the only unpaired electron. In contrast,
2.5
2.0
6
8
10
12
14
16
18
20
n in C,H,(NO),’ Fig. 10. Ratio of the integrated clusters size n.
intensities
of the two absorption
features assigned as Y, and vs. The graph shows Y$Y, plotted as a function
of
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odd-sized cluster ions, e.g. (NO);,, may adopt the form (NO);.[(N0)214 with no unpaired electrons and, therefore, a less significant influence on the v8 vibrational mode of an attached ethylene molecule. Additional, circumstantial evidence in support of individual elements contained within the proposed configurations, is to be found in a wide variety of other cluster and condensed phase experiments [8,11- 14,22-251. 4. Conclusion Results have been presented of a study of the infrared photofragmentation patterns and spectra of C2H4*(NO)n+ cluster ions. The decay patterns (metastable and laser-induced) show an interesting alternation as a function of size, which we attribute to differences in stability between oddand even-electron ions. In addition to exhibiting an absorption profile due to an infrared allowed transition, v7, the clusters also show evidence of a feature that we have tentatively assigned as due to the vg vibrational mode which, in the isolate ethylene molecule, is Raman active. The v8 mode appears to become IR active in clusters that contain a single unpaired NO molecule, and it is suggested that an interaction between the odd electron on NO and the LUMO of ethylene may be responsible. An obvious check will be to study the spectroscopy of the neutral C2H4.N0 complex under high resolution conditions. Acknowledgements The authors would like to thank EPSRC for financial support and AM would like to thank the Atomic Energy Commission of Syria for a research studentship.
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References [l]
CA. Woodward, J.F. Winkel, A.B. Jones and A.J. State, Chem. Phys. Lett., 49 (1993) 206. [2] A.B. Jones, C.A. Woodward, J.F. Winkel and A.J. State, Int. J. Mass Spectrom. Ion Processes, 133 (1994) 83. [3] J.F. Winkel, A.B. Jones, CA. Woodward, D.A. Kirkwood and A.J. State, J. Chem. Phys., 101 (1994) 9436. [4] J.F Winkel, CA. Woodward, A.B. Jones and A.J. State, J. Chem. Phys., 103 (1995) 5177. [S] A.J. State, A.B. Jones, J.F. Winkel and A.J. State, J. Phys. Chem., 97 (1993) 11363. [6] P.G. Lethbridge and A.J. State, J. Chem. Phys., 89 (1988) 4062. [7] CA. Woodward and A.J. State. J. Chem. Phys., 94 (1991) 4234. [8] S. Atrill, A. Mouhandes, J.F. Winkel, A. Goren and A.J. State Faraday Dicuss., 102 (1995), in press. [9] A.B. Jones, R. Lopez-Martens and A.J. State, 3. Phys. Chem., 99 (1995) 6333. [lo] R.G. Cooks, J.H. Beynon, R.M. Caprioli and G.R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973. [ll] H.S. Carman, Jr., J. Chem. Phys., 100 (1994) 2629. [12] C.-Y. Kung, R.A. Kennedy, D.A. Dolson and T.A. Miller, Chem. Phys. L&t., 145 (1988) 455. [13] H.S. Carman, Jr., S.R. Desai, C.S. Feigerle, J.C. Miller. In: L.A. Bloomfield, T.G. Gallagher and D.J. Larson (Eds.), Laser Spectroscopy XI (Proc. 11th Int. Conf. on Laser Spectroscopy), American Institute of Physics, New York, 1994, p. 151. (141 A. Mouhandes and A.J. State, to be published. [15] M. Cassassa, D. Bomse and K. Janda, J. Chem. Phys., 74, (1981) 5044. [ 16) M. Hoffbauer, K. Liu, C. Giese and W. Gentry, J. Chem. Phys., 78 (1983) 5567. [ 171 H. Godfried and I.F. Silvera. Phys. Rev., A27 (1983) 3008 and 3019. [ 181 J. Geraedts, Infrared Excitation of Clusters. PhD. Thesis, University of Nijmegen, 1983. [19] S. Atrill and A.J. State, to be published. (201 J. Geraedts, S. Stoke and J. Reuss, Chem. Phys. Lett., 97 (1983) 152. [21] J.L. Duncan and E. Hamilton, J. Mol. Struct., 76 (1981) 65. [22] H.L. Johnston and W.F. Giauque, J. Am. Chem. Sot., 51 (1929) 3194. [23] O.K. Rice, J. Chem. Phys.. 4 (1936) 367. [24] A.L. Smith, W.E. Keller and H.L. Johnston, J. Chem. Phys., 19 (1951) 189. (251 A. Anderson and B. Lassier-Govers, Chem. Phys. Lett., 50 (1977) 124. 1261 L. Salem, The Molecular Orbital Theory of Conjugated Systems, W.A. Bejamin, MA, 1972.
International Journal of Mass Spectrometry and Ion Processes 1.59(1996) 185-196
Infrared photofragmentation spectra of C2H4.(NO)L cluster ions A. Mouhandes, A.J. State” School
of Molecular
Sciences,
University
of Sussex,
Falmer,
Brighton,
BNI
9QJ, UK
Received 19 April 1996; accepted 10 June 1996
Abstract A line-tuneable CO2 laser has been used to excite one or more vibrational modes in ethylene molecules resident on sizeselected C2HJ(NO)i cluster ions, for n in the range 2-20. Excitation is followed by the observation of three separate fragmentation channels: -C2H4, -NO and -2N0, whose relative intensities are found to be extremely sensitive to cluster size. The most marked transitions in fragmentation pattern are found to occur between odd- and even-sized cluster ions, with decay channels favouring the formation of even electron ions. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles. Those profiles associated with several of the even-sized ions show evidence of two prominent absorption features, one of which is attributed to the v, infrared active vibrational mode in ethylene. A second absorption feature is tentatively assigned as the yg mode which, under normal circumstances, is Raman active; however, a mechanism is proposed whereby the ethylene molecule could become distorted. Keywords:
Fragmentation
channels; Infrared absorption profiles
1. Introduction In a recent series of experiments, we have demonstrated that, through a careful selection of cluster-chromophore combinations, it is possible to study the spectroscopy and decay dynamics of single neutral infrared chromophores in association with mass-selected cluster ions [l-4]. The advantage offered by these experiments is that it then becomes possible to correlate the evolution of spectral features with other experimental observables, such as mass spectra and the decay patterns of photoexcited cluster ions. Thus, changes in absorption frequency can be equated with the presence of ‘magic numbers’ and/or the introduction of new * Corresponding author.
fragmentation routes as identified in a mass spectrometer. The key aspect of these experiments is that, in heterogeneous clusters of the form XY,, the chromophore X should have an ionisation energy (IE) which is larger than that of the host cluster Y,. Under these circumstances, the cluster can be ionised in the knowledge that it is Y, which carries the positive charge [l-4]. Only under very unusual circumstance do we observe infrared-induced charge transfer between the chromophore and the host cluster [5]. A disadvantage of the approach discussed here is that there is only limited control over the internal energies of the cluster ions. Electron impact ionisation is a comparatively violent process and capable of creating a broad range of excited states; however, because observation time scales are of the order of 10-5-10-4 s, many of these
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of Mass Spectromettyand
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states decay to leave a residual vibrational energy in the ground electronic state. This energy can then promote the break-up of cluster ions during the course of an experiment. Since unimolecular decay is still observed after 10e4 s [6], it is known that a certain fraction of cluster ions possess an internal energy which is, at least, equivalent to the minimum dissociation energy, Ei. However, because gas cluster ions of the type discussed here are, for the most part, weakly-bound, their internal energies cannot be very much higher than Ei, otherwise extensive fragmentation would occur prior to reaching the point of laser excitation. Hence, the energy deposited from an IR photon (-0.11 eV) is a significant fraction of the total internal energy and, as a consequence, photofragmentation signals are almost always found to be very much stronger than those recorded for metastable decay [l-4]. An estimate of average internal energy can be obtained from the measurement of kinetic energy releases associated with unimolecular decay [7,8].
Magnet Laser Port
,.
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Reported here are the results of a series of measurements undertaken on the IR photodissociation spectroscopy of C,H,*(NO)i clusters for n in the range 2-20. From a summation of fragment ion intensities it has been possible to derive vibrational absorption spectra as a function of cluster size, and these are used to discuss the structures of the ions. Both the fragmentation patterns and the recorded absorption profiles exhibit features not seen in related experiments on other cluster ion systems.
2. Experimental
section
Mixed neutral clusters of the form C2H4*(NO)n were prepared using the “pick up” technique on an apparatus that consists of a supersonic nozzle coupled to a modified double-focusing “reverse geometry” VG ZAB-E mass spectrometer. Fig. 1 shows a schematic diagram of the apparatus and details of the exact construction have been given
i Magncttc
159
i- .
ESA Laser Port
:. ..
Vacuum Generator ZAB-E, reverse geometry double focusing mass spectrometer hzieE+
c :,
6
” .c”
Nozzle CO2 Laser
Ion Source
Cluster Chamber
Fig. 1. Schematic
diagram
of the apparatus
A. Mouhandes,
A.J. Stace/Intemational
Journal
of Mass
previously [6]; the only additional feature being a line-tuneable CO* infrared laser (Edinburgh Instruments PL4). Neutral clusters of NO were generated via the adiabatic expansion of a mixture consisting of 1% nitric oxide in argon through a 200 pm diameter pulsed conical nozzle. Following collimation through a 1 mm diameter skimmer, the cluster beam entered a flight tube approximately 70 cm long, in which the background pressure had been increased to -lo-’ mbar through the introduction of C2H4 via a needle valve. During their passage through this region, some of the NO clusters formed collision complexes with C2H4 molecules and optimum ion signals were observed when the nozzle stagnation pressure was approximately 60 psi. Ionisation to C2H4.(NO)n+ was achieved using 70 eV electrons, and use of the “pick-up” procedure [l-4] proved very successful in generating stable ion currents with almost uniform intensity for clusters consisting of a single chromophore and up to twenty host molecules. One disadvantage of using an nitric oxide-argon mixture is the high probability of forming mixed clusters. Although the masses of clusters can, for the most part, be identified separately, there is the problem of a mass equivalence between four NO molecules and three Ar atoms (120 amu); thus large C2H4(NO)n+ clusters could contain some argon atoms and still have the same nominal mass. Experiments showed that, following photoexcitation, the mixed clusters gave preferential loss of argon rather than NO. Hence, the decay patterns of pure C,H,(NO),’ clusters could be monitored accordingly; however, parent ion intensities could not be used to normalise the data, instead metastable (unimolecular) peak intensities (see below) were used for this purpose. Experiments with a range of vapour-gasargon mixtures [2,3,9] suggest that the presence of argon atoms in the clusters immediately following adiabatic expansion, plays an important part both in the C2H4 attachment process and in stabilising the ion complex following ionisation.
Spectrometry
and
The initial the form: (NO);Ar,
-
Ion Processes
159
(1996)
“pick-up”
collision
+ C2H, -
[C&W&J&,]
C2H,*(NO),
185-196
probably
187
takes
+ mAr
where [C2H,(NO),;Ar,]# denotes a collision complex which is ultimately stabilised through the evaporation of the argon atoms and, possibly, some of the nitric oxide molecules. The argon atoms would depart first since, of the species present, they should have the lowest binding energy; a similar stabilisation step probably accompanies ionisation. Attempts to perform “pick-up” experiments in the absence of argon, for example, using expansions of pure NO or COZ, have always resulted in failure [2,3]. Ions were extracted from the source with a potential of 7 kV and size-selected using a magnetic sector. These ions then entered a field-free region where they were irradiated with photons from a line-tuneable carbon dioxide laser (frequency range 920-l 100 cm-‘, resolution -1.5 cm-‘), which was aligned coaxially with the ion flight path; the overlap between the laser and ion beams covered a distance of approximately 150 cm. The masses of any ionic photofragments were determined using the electrostatic analyser in the MIKES (mass-analysed ion kinetic energy spectra) mode [lo]. To eliminate possible interference from fragment ions due either to metastable decay (see below) or collision-induced dissociation (CID), the laser beam was modulated at half the nozzle frequency and foreground-background subtraction performed on photofragment signals. Two data collection techniques were employed: photodissociation and unimolecular decay signals at a.single kinetic energy were recorded using gated photon counting via a scintillation (Daly) ion detector; mass spectra were recorded using phase-sensitive detection in conjunction with analogue output from the ion detection system. To keep CID processes to a minimum, the background pressure in the flight tube was
188
A. Mouhandes,
A.J. Stacellnternational
Journal
of Mass Spectromerr)
and Ion Processes
159 (1996)
185-196
Metastable decay pattern of C,H4(NO)n’ loss of NO I ’ * ”
1”
n ’ u ’ n ’ n ’ u ’ ”
1
1.0
/
0.0 I,
1
2
4
t
I,
I.I
6
8
I
I,
I.
IO
12
II1I.I I
I4
I,
I
16
18
‘1
I
20
n in C,H,(NO),+
(a)
Metastable decay pattern of C,H,(NO),C
0.8
loss
of 2N0
n
h
.3
0.0
(4
I, 4
I1 6
10 8
I1 10
I. 12
I 14
I
I 16
I
I1 18
I 20
n in C,H,(NO),+
Fig. 2. Observed metastable fragmentation patterns for C2H4(NO)i cluster ions plotted as a function been plotted separately in order to emphasise the odd-even alternation in signal intensity.
of n. The two fragmentation
channels have
A. Mouhandes,
A.J. StaceJlntemational
of Mass Spectrometry
Journal
maintained at 6 x 10e9 mbar. Photodissociation signals were between lo4 and 10” counts min-’ against a background signal of
3. Results and discussion 3.1. Metastable
decay pattern
Following mass selection, three separate unimolecular (metastable) decay routes were identified for C2H4(NO)i cluster ions: C2H4.(NO);
-
C2H, + (NO);
C,H,.(NO);
-
C,H,.(NO);-
(1) i + NO
n>3 (2)
C2H4*(NO)n+ -
CzH4.(NO)&
+ 2N0
n>3
(3) Note the very specific nature of reaction (1); this decay route is not exhibited by any other size of cluster. In an earlier study of SF6(NO),’ the unimolecular loss of SF6 was also found to be size-specific, but not to the degree seen here [3]. Fig. 2 shows the relative intensities of
and Ion Processes
I59 (1996)
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189
fragment ions from the two metastable decay routes (2) and (3) above, plotted as a function of cluster size. The marked alternation in behaviour between odd- and even-sized clusters shown for C2H4*(NO)i is quite pronounced and follows a pattern that is mirrored by the parent ion intensities. The fragmentation patterns recorded for SF,(NO)i clusters also exhibited alternations of the type shown in Fig. 2 and, the fact that similar patterns of intensity fluctuation are also seen in the mass spectra of both (NO): and SF,(NO)’ clusters [3,11-131, suggests that the phenomenon is determined by the stability of the underlying (NO); moiety. The NO molecule is unusual in that the ground-state electronic configuration
-tm
(~2~)~
(~~J2
@2$
(nJ4
G.pl
gives it a single unpaired electron in a a antibonding HOMO. In contrast, NO’ is an evenelectron ion (‘C’) with all the valence electrons present as pairs. We believe this characteristic extends to (NO);, and that the particular stability of this closed-shell ion accounts for reaction (1) and also results in (NO); acting as a central core for larger positively charged ions. Such a core would have to be an odd-sized ion in order to explain the decay pattern observed for the larger clusters in Fig. 2 (see below). These assumptions regarding (NO); are supported by separate CO (carbon monoxide) laser photofragmentation studies of SF,(NO),’ and (NO),’ clusters, which both show strong evidence of (NO),’ being a stable infrared chromophore w41. If the above picture is now extended to the larger clusters, then it can be seen that all oddsized C2H4(NO)i clusters will be even electron systems; these are the most intense ions and they prefer to lose 2 NO molecules in order to maintain the stability associated with n = odd. In contrast, even-sized C2H4(NO),’ are odd electron systems; these are at least an order of magnitude less intense than n = odd, and they exhibit a strong preference for the loss of just one NO molecule in order to generate stable evenelectron clusters.
190
A. Mouhandes,
A.J. Staceilnternational
Journal
of Mass Spectrometly
and Ion Processes
159 (1996)
185-196
Photoinduced loss of NO for C,H,(NO),’
0.8
2
4
6
8
IO
12
14
16
18
20
n in C,H,O\IO),+
(4
Photoinduced loss of 2N0 for C,H,(NO),+
0.8
0.2
0.0
(b) Fig. 3. Observed photoinduced fragmentation mately 967 cm-’ and the two fragmentation intensity.
n in C,H,(NO),’ patterns for C2H4(NO).’ cluster ions plotted as a function of n. The photon energy is approxichannels have been plotted separately in order to emphasise the odd-even alternation in signal
A. Mouhandes,
A.J. Stacellnternational
3.2. Infrared photofragmentation profiles
Journal
of Mass Spectrometry
patterns and
and ton Processes
159 (1996)
185-196
191
small fraction of the photon’s energy is dissipated into the cluster ion. In addition to the IR active mode, ethylene has a second mode, vg, at 940 cm-‘, which is Raman active. There has been a suggestion that following the formation of a neutral ethylene dimer, the latter mode may acquire infrared activity as a consequence of a v7-vg Fermi resonance [18,20]. In the case of C2H4*(NO)n+ cluster ions, the magnitudes of any shifts and the degree of interaction between modes may well be accentuated by virtue of the presence of a positive charge; evidence to this effect is to be found in a recent study of SF,.Ari cluster ions [4]. Fig. 3 shows the relative intensities of the photo-induced signals recorded for reactions (2) and (3). As might be expected, the fragmentation pattern follows that given for metastable or unimolecular decay; the reason being that a single infrared photon does not add a significant amount of extra energy to an ion. In effect, the factor(s)
Within the range of the CO1 laser, CzHJ has one infrared active mode which is v7 (Br,) at 948.8 cm-‘. In association with a broad range of atoms and molecules, the infrared predissociation of neutral ethylene dimer complexes has been the subject of very extensive study [1518]. In the experiments discussed here, the photo-induced decay mechanism is considered to be very similar to that identified in earlier work: C2H4 absorbs a photon and after some delay, a fraction of that energy dissipates into the cluster ion where it is used to promote dissociation. Recent studies of kinetic energy release following the decay of photoexcited SF&NO),+ [S] and SF6*Ari [19] cluster ions have shown that, following absorption of an infrared photon, the SF6 molecule undergoes state-to-state vibrational relaxation and only a C,H,(NO),+
loss of NO
1.2
1.0
.s 0.8 s f <
0.6
. 2 *z d 3
0.4
0.2
00 930
940
950
960
970
980
990
Photon Energy / cm-’ Fig. 4. Infrared absorption profile recorded wavelength. The points are the experimental latter are given as broken lines.
for C2H4.(N0)i and determined by summing all fragment ion intensities as a function of laser data and the solid line is a sum of the fitted Lorentzian profiles, where individual components to the
192
A. Mouhandes,
A.J. Stacellnternational
Journal
of Mass
Spectrometly
responsible for the pronounced fluctuations in fragment ion signal outweigh the perturbation imposed by the addition of a ~950 cm-’ photon. From a summation of photofragment ion intensities as a function laser wavelength, it has been possible to derive infrared absorption profiles, and examples of these are shown in Figs. 4-8. For each example the profiles have been fitted to Lorentzian line shapes where the parameters were position of the peak maximum and FWHM. In all cases, a least squares analysis of the residuals between experimental data and fitted profiles, gave an average error of Sloe2 on a scale where the data points are normalised to 1. Attempts to fit the data to Gaussian profiles always gave errors >>10e2. In a majority of cases, the profiles consist of two relatively intense peaks centred at ~957 cm-’ and ~966 cm-‘, respectively, together with minor contributions which appear at ~945 cm-’ and ~990 cm-‘, but do not follow any systematic pattern. Fig. 9 presents a summary of wavelength
I
loss of NO
I
1.0 1.0 -
~ .z c 7
159 (1996)
185-196
variations seen for the two principal contributions to each profile, plotted as a function of cluster size. The plot shows two quite distinct absorption frequencies which are almost independent of cluster size and separated throughout the range by approximately 9 cm-‘. It is interesting to note that the frequency difference between v7 and vs in isolated ethylene is 8.8 cm-’ [21]. If the observed features do correspond to v7 and v8, then both are blue-shifted by approximately 15 cm-l with respect to the isolated molecule and, although such shifts appear quite frequently in these experiments [l-4], the above values are slightly larger than those seen previously. The widths of the two absorption features identified in Fig. 9 vary between 4 and 19 cm-’ which, although consistent with previous measurements on neutral ethylene complexes [15-181, show no obvious trends as a function of cluster size. One of the particular features of this experiment is that the cluster ions are mass- or
C,H,(NO),+ I
and Ion Processes
I
I
I
.
0.8 -
I
I
I
1
I
I
940
950
960
970
980
990
Photon Energy / cm-’ Fig. 5. As for Fig. Fig, 4, but for C2H,(NO);.
_
A. Mouhandes,
A.J. StaceJlnternational
Journal
of Mass Spectrometry
and Ion Processes
I59 (1996)
C,H,(NO),,+ loss of NO
0.8
0.6
0.0 1
’
930
I 940
,
I
I
I
I
I
950
960
970
980
990
Photon Energy / cm-’ Fig. 6. As for Fig. 4, but for C2H,(NO);,,.
C,H,(NO),,+ loss of NO 12
10
I
I
I
I
I
I
-
0.2 -
0.0 930
’
I
I
I
I
1
I
940
950
960
970
980
990
Photon Energy / cm-’ Fig. 7. As for Fig. 4, but for C2HJ.(N0);2.
185-196
193
194
A. Mouhandes,
A.J. Stacellntemational
Journal
of Mass Spectrotnetry
and Ion Processes
159 (1996)
185-196
C,H,(NO), ,+ loss of 2(NO) I
I
I
1
A .’,‘.. ., .
0.8 vl .Z 5 e
0.6
2
.
.e $ z -
0.4
,.:.u ‘,.,
l
‘.
0.2
.’
,‘.
‘.
,’
.’
‘._.
_’
:.
_:
.:_
‘,
‘.
‘.
:
: .-.
:.:
...
.
._
:..i.Y;..:i
0.0
930
940
950
960
970
.
980
. .
990
Photon Energy / cm-l
Fig. 8. As for Fig. 4, but for C?H4$NO);,.
950 2
1,
I,
,,I,,
4
6
8
, 10
12
,
,
14
, 16
,
, 18
, 20
n in C,H,(NO),’ Fig. 9. Peak centres of the two principal
absorption
features
seen in each of the profiles,
plotted
as a function
of cluster size n.
A. Mouhandes,
AJ. Stacelintemational
Journal
of Mass Spectrometry
size-selected; hence we can be quite certain that the ions do not contain more than one C2H4 molecule. Therefore, the features seen in Figs. 4-8 cannot be equated with two molecules on the same cluster occupying different sites. The only obvious possibilities are: (i) two distinct types of C2H4*(NO)i cluster are being generated for each value of 12and, in each case, the resident CZHa molecule is located in one of two possible positions that are sufficiently different as to cause a relative shift of 9 cm-’ in the v7 mode; (ii) more than one IR active mode falls within the wavelength range scanned by the laser. An interesting relationship is to be found if the ratio of the integrated intensities for the two absorption features shown in Fig. 9, is plotted as a function of cluster size. That result is given in Fig. 10, where it can be seen that there is a marked variation with cluster size, with the component at lower frequency (possibly vs) being the more intense on even-sized cluster ions. As discussed earlier, even-sized (NO),’ ions will contain an odd number of electrons and,
and Ion Processes
I59 (1996)
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195
because evidence from condensed phase experiments supports the formation of stable NO dimers [22-251, large even-sized cluster ions might be expected to contain, on average, a single unpaired NO molecule and, as a consequence, an unpaired electron. Under those circumstance, any interaction between the latter and the ethylene molecule is most likely to proceed via the r antibonding LUMO orbital on C2H4. From UV electronic spectra (an extreme case) [26], it can be seen that any introduction of electron population into the b, antibonding orbital on ethylene results in considerable distortion to the molecule. Thus, any interaction between a single NO and C2H4 can be expected to distort the latter, at which point the vs vibrational mode should become infrared active. From Fig. 10, it can be seen that what we assume to be v8 acquires considerable intensity in the even-sized cluster ions which, for a cluster like (NO);,,, suggests a configuration of the form: (NO)$[(NO)&NO, where the single nitric oxide molecule contributes the only unpaired electron. In contrast,
2.5
2.0
6
8
10
12
14
16
18
20
n in C,H,(NO),’ Fig. 10. Ratio of the integrated clusters size n.
intensities
of the two absorption
features assigned as Y, and vs. The graph shows Y$Y, plotted as a function
of
196
A. Mouhandes,
A.J. Stacellnternational
Journal
of Mass Spectrometry
odd-sized cluster ions, e.g. (NO);,, may adopt the form (NO);.[(N0)214 with no unpaired electrons and, therefore, a less significant influence on the v8 vibrational mode of an attached ethylene molecule. Additional, circumstantial evidence in support of individual elements contained within the proposed configurations, is to be found in a wide variety of other cluster and condensed phase experiments [8,11- 14,22-251. 4. Conclusion Results have been presented of a study of the infrared photofragmentation patterns and spectra of C2H4*(NO)n+ cluster ions. The decay patterns (metastable and laser-induced) show an interesting alternation as a function of size, which we attribute to differences in stability between oddand even-electron ions. In addition to exhibiting an absorption profile due to an infrared allowed transition, v7, the clusters also show evidence of a feature that we have tentatively assigned as due to the vg vibrational mode which, in the isolate ethylene molecule, is Raman active. The v8 mode appears to become IR active in clusters that contain a single unpaired NO molecule, and it is suggested that an interaction between the odd electron on NO and the LUMO of ethylene may be responsible. An obvious check will be to study the spectroscopy of the neutral C2H4.N0 complex under high resolution conditions. Acknowledgements The authors would like to thank EPSRC for financial support and AM would like to thank the Atomic Energy Commission of Syria for a research studentship.
and Ion Processes
159 (1996)
185-196
References [l]
CA. Woodward, J.F. Winkel, A.B. Jones and A.J. State, Chem. Phys. Lett., 49 (1993) 206. [2] A.B. Jones, C.A. Woodward, J.F. Winkel and A.J. State, Int. J. Mass Spectrom. Ion Processes, 133 (1994) 83. [3] J.F. Winkel, A.B. Jones, CA. Woodward, D.A. Kirkwood and A.J. State, J. Chem. Phys., 101 (1994) 9436. [4] J.F Winkel, CA. Woodward, A.B. Jones and A.J. State, J. Chem. Phys., 103 (1995) 5177. [S] A.J. State, A.B. Jones, J.F. Winkel and A.J. State, J. Phys. Chem., 97 (1993) 11363. [6] P.G. Lethbridge and A.J. State, J. Chem. Phys., 89 (1988) 4062. [7] CA. Woodward and A.J. State. J. Chem. Phys., 94 (1991) 4234. [8] S. Atrill, A. Mouhandes, J.F. Winkel, A. Goren and A.J. State Faraday Dicuss., 102 (1995), in press. [9] A.B. Jones, R. Lopez-Martens and A.J. State, 3. Phys. Chem., 99 (1995) 6333. [lo] R.G. Cooks, J.H. Beynon, R.M. Caprioli and G.R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973. [ll] H.S. Carman, Jr., J. Chem. Phys., 100 (1994) 2629. [12] C.-Y. Kung, R.A. Kennedy, D.A. Dolson and T.A. Miller, Chem. Phys. L&t., 145 (1988) 455. [13] H.S. Carman, Jr., S.R. Desai, C.S. Feigerle, J.C. Miller. In: L.A. Bloomfield, T.G. Gallagher and D.J. Larson (Eds.), Laser Spectroscopy XI (Proc. 11th Int. Conf. on Laser Spectroscopy), American Institute of Physics, New York, 1994, p. 151. (141 A. Mouhandes and A.J. State, to be published. [15] M. Cassassa, D. Bomse and K. Janda, J. Chem. Phys., 74, (1981) 5044. [ 16) M. Hoffbauer, K. Liu, C. Giese and W. Gentry, J. Chem. Phys., 78 (1983) 5567. [ 171 H. Godfried and I.F. Silvera. Phys. Rev., A27 (1983) 3008 and 3019. [ 181 J. Geraedts, Infrared Excitation of Clusters. PhD. Thesis, University of Nijmegen, 1983. [19] S. Atrill and A.J. State, to be published. (201 J. Geraedts, S. Stoke and J. Reuss, Chem. Phys. Lett., 97 (1983) 152. [21] J.L. Duncan and E. Hamilton, J. Mol. Struct., 76 (1981) 65. [22] H.L. Johnston and W.F. Giauque, J. Am. Chem. Sot., 51 (1929) 3194. [23] O.K. Rice, J. Chem. Phys.. 4 (1936) 367. [24] A.L. Smith, W.E. Keller and H.L. Johnston, J. Chem. Phys., 19 (1951) 189. (251 A. Anderson and B. Lassier-Govers, Chem. Phys. Lett., 50 (1977) 124. 1261 L. Salem, The Molecular Orbital Theory of Conjugated Systems, W.A. Bejamin, MA, 1972.