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Ion internal energy selection through angle-resolved mass spectrometry
International Journal of Mass Spectrometry and Zon Physics, 33 (1980) 231-241 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
ION INTERNAL ENERGY MASS SPECTROMETRY
J.A.
LARAMEE,
Department (Received
P.H.
HEMBERGER
of Chemistry, 5 July
SELECTION
and R.G.
Purdue University,
THROUGH
231
ANGLE-RESOLVED
COOKS
West Lafayette,
IN 47907
(U.S.A.)
1979)
ABSTRACT Collision-induced dissociation of kilovolt energy ions gives product ions whose nature and abundance depend on the scattering angle. Increasing angle corresponds to depositing increased internal energies. The angle-resolved data for methanol, ethanol, propsn-l-01, methane, and propane closely parallel the breakdown curves for these molecular ions. Fragmentations from isolated electronic states are suggested by the angle-resolved data for CS’,-. The results provide further evidence that collisional activation at kilo-electron volt energies occurs by direct 4ectronic excitation. This methodology is also shown to allow the effectivene.ss of different targets in collisional activation to be determined.
INTRODUCTION
Control over ion internal energy is desirable in many mass-spectral studies, particularly in kinetics, but is relatively difficult to achieve. The classical procedure is charge exchange with a rare gas ion [1,27. More recently, photoelectron/photoion coincidence spectroscopy has emerged as a powerful procedure for studying ions with well-defined internal energies [ 3,4]. The selection of ions with varying internal energies has been important in studies on the dissociations of organic ions. Thus rather crude energy selection has been obtained, for example, by comparing dissociations occurring in different regions of the mass spectrometer [ 51 or by comparing spontaneous and collision-induced dissociations [ 61. Field ionization kinetics has extended the accessible range of lifetimes and hence of internal energies ]73_ A number of physical properties vary with ion internal energy, among them kinetic energy release [S-lo], isotope effects 111,121 and branching ratios of competitive fragmentation channels [13]_ Each of these properties is useful in characterizing ion structure. In this paper we explore the possibility that approximate selection of internal energy can be achieved by definition of the scattering angle in an ion/neutral target collision. In all cases the dissociations which ensue from a high energy (keV range) collision are followed as a function of the scattering angle of the product ion. If a correspondence can be assumed between the
232
impact parameter for a collision and the internal energy deposited - hard collisions depositing more energy than soft collisions - then selection of the scattering angle should represent a selection of ion internal energy. This hypothesis has been tested by measuring the collision-induced dissociations (CID) undergone by particular ions as a function of scattering angle. The relative product ion abundances measured as a function of angle arc compared to relative ion abundances as a function of internal energy, viz. to the breakdown curves for the ions in question. It has previously been shown 114,151 that internal energies well in excess of 10 cV can be deposited in organic ions in kilovolt energy collisions at glancing angles. It has also been shown by exact measurements of product ion kinetic energies that the energy loss in the collision (approximately the cndothermicity of the reaction for fast ions, thermal targets and near-zero scattering angles) correlates with the activation energy for the process in question. In particular, for a series of alcohol molecular ions the energy loss associated with CID is approximately equal to that internal energy known from breakdown curves to maximize the generation of the analyzed product ion 1141. The alcohols thus seem to behave on high energy collision analogously to their behavior on electron impact. This is consistent with the view that electronic excitations are dominant in both processes 1161 and led us to select these species for the present study, in which collisional activation is probed further. EXPERIMENTAL
A Hitachi RMH-2 mass spectrometer modified for angle distribution studies on ion/molecule reactions occurring in the second field-free region was used 1173. Operating conditions were 8-kV accelerating voltage, an electron ionizing energy of 70 eV, and an electron current of 1 mA. Accelerating voltage and lens potentials were adjusted in order to position the reactant beam symmetrically in the collision chamber. Either argon or nitrogen collision gas was used. The collision gas pressure was chosen to optimize the signal strength of the collision-induced dissociation of interest. In all cases the gas pressure measured in the first field-free region using an ion gauge was l-2 X 10S6 torr, corresponding to approximately 10 mtorr in the collision chamber. The energy-resolving slit width was 0.31 mm and the a-slit width was adjusted to attenuate the beam by a factor of 2. These conditions gave an angular spread (in the plane of analysis) of the unreacted primary ion beam in the absence of target gas of +0.07” at half maximum with 98% of the ions being transmitted in this range. The angle-resolving slit width was set at 1.25 mm so that the range of scattering angles about the central ray selected for transmission in the plane of analysis . (x-y plane) was usually iO.08” and in the XYZ plane it was *0.25”. Convolution of the experimental beam divergence (+0.07”) with the geometrically determined range of scattering angles (+0.08” ) yields an angular resolution
233 TABLE
1
Estimated precision of measurements Scattering angIe (“) *
Precision (o/o)
0.1 0.5 1
+ 0.5 f2 i10
* Angle measured in the x--y plane.
of t0.1" (x-y plane). Where indicated, higher resolutions were employed; in one experiment the angle-resolving slit was 0.63 mm giving iO.04” scattering angle in the xy pl*anc and the collector slit height was decreased from 10 mm to 2 mm, corresponding to selection of X-Z angles of ;-0.05”. This improved resolution had the expected effect of sharpening the features seen in the lower resolution spectra (compare Fig. 5). A significant deterioration in signal-to-noise ratio occurred as the scattering angle was increased. The totaI CID product abundance at 1” (laboratory) was typically 1% of the value at O”. This deterioration is primarily due to the decreased cross-section for scattering through larger angles. The variation in collection efficiency is not thought to be significant, especially since the position of the magnet was adjusted to optimize transmission of scattered products. The relative abundance precision versus angle is estimated in Table 1. This table applies to all angle-rcsolvcd vaIues presented except for the high-resolution CSz data. Collision-induced dissociations were assigned on the basis of the momentum of the product ions which correspond approximately to an apparent mass of n&n, for the transition rn: -s rnz + m+ The exact momentum is 3 little less than this value owing to cncrgy loss in the collision. At larger angles both the inelastic and the elastic energy loss components increase so that a slight decrease in magnet field strength was needed to keep 3 particular process in focus as the scattering angle was increased. This factor limited the usefulness of a method of taking data which simply involved scanning the angle-resolving slit while detecting the products of a particular reaction. A slower procedure in which complete momentum (mass) spectra were taken at selected scattering angles was therefore generally used. RESULTS AND DISCUSSION The angle-resolved CID spectra of various molecular ions have been obtained using laboratory scattering angles of O-l” and an angular resoluiion of i-0.1” as described above. The relative intensity of the CID transitions as a function of the scattering angle is displayed in Figs. L-5 for some of the systems investigated. Shown for comptiison arc the theoretical breakdown
234
curves calculated [ 181 by quasi-equilibrium theory (QET) and in one case (Fig. 2) the experimental charge exchange data [ 191. Agreement of the overall features of both sets of curves is excellent. This establishes that, at least qualitatively, ion internal energy can be selected by varying the scattcring angle. The size of the data points in the figures is not indicative of the precision of measurement, which decreases with increasing scattering angle. (Signal strengths decreased by a factor of 100 on reaching the higher angle settings.) Where necessary, corrections were made for the contribution of unimolecular dissociation (metastable ions) to the collision-induced dissociation process. This was accomplished by measuring the intensity of the signal of the metastable ion at various angles with no collision gas in the second fieldfree region. In comparing the breakdown curves with the angle-resolved spectra, note that the minimum energy accessible in the latter corresponds to the onset of collision-induced dissociation. Truncating the breakdown curves at a non-zero energy (broken lines) therefore provides the best comparison of the two sets of data. The experimental results for each of the molecular ions may now be discussed in turn. Propan-I-ol (m/z 59) is the most facile process The dissociation of M” to (M -H)* sampled at zero-scattering angle. The rapid depletion of this ion with increasing scattering angle is associated with the opening of several reaction chanI - PROPANOL
ENERGY
bV)
.
k?
COLLISION
GAS
ANGLE
(d.Qrsd
Fig. 1. Comparison of the theoretical breakdown curve of the molecular ion of propanl-01 [lS] with the collision-induced dissociations of this ion recorded as a function of laboratory scattering angle. All data are given as relative abundance. The dashed line indicates the position on the breakdown curve corresponding to the zero-angle CID results. At lower energies the molecular ion dominates and there is no parallel in the angleresolved data.
236
METHANOL.
COLLISION
GAS
OET CALCULATION
60t!! 3
Ng
=O
Cl0
(\
2 Q IiJ
40-
= !i 2 az
20-
0
2
4
6
ENERGY
]
m’
cl2
04
06
ANGLE
08 kJegrss)
23
I--._.
I2
0
CHARG~I EXCHANGE
k IO
6 (avl
14
ENERGY
3!?.../
I6
IS
(SW
Fig. 2. Breakdown curve 1161 for the methanol reso!vcd CII3 and charge exchange data [ 17 1.
molecular
ion
compared
with
angle-
nels for fragmentation of (M - H)‘. The theoretical breakdown curve shows ions 59’ and 29*, (C,H,)‘, to be of equal abundance at the maximum for 31’, (CH,OH’) (Fig. 1). This featur,p is reproduced in the angle-resolved CID curve. Agreement of the two curves is preserved at high internal energy and large scattering angle. (Data for 27’ are not given in ref. 18.) Me thanoi Many of the features of propan-l-01 are preserved in the methanol plot (Fig. 2), for example, the rapid decrease of (M - H)’ and the increasing rela-
236
1 90
PROPANE,
-
N2
toU.lSlON
GAS
29
4
2
0
9
6 ENERGY
Fig. 3. Breakdown curve [16] resolved CID data.
a9
Cl6
04
0.2
0
IO
ANGLE
fcV)
(degree)
for the propane molecular ion compared with the angie-
tivc abundance of 29’ at high angle. The elimination of Hz from the methanol molecular ion is a minor process at any angle. The agreement among the breakdown curve, the charge exchange data, and the angular-resolved curves is good. It is particularly noteworthy that the two experimental procedures give closely corresponding results. Propane Although the changes in relative abundance of the ions in the CID curve (Fig. 3) are not nearly as sharp as those predicted by QET, good qualitative
\:
METHANE.
>
15
90.
60
-
40
-
COLUSION
GAS
.--_ ,_
-‘-A. _/._____.
\
I5
>
20
14
1
Al
__--
‘\.___.....
14
.,-‘.
~
i
02’ ENLRGY
Fig. 4. Breakdown curve resolved di.ssociations.
[ 16 ] for the mothane
molecular
08
OG
04 ANGLE
WI
10
(degree)
ion compared
to the angle-
237 TAl3LE
2
Collision-induced Laboratory angle (“)
dissociation
of C2HSOH+-
as a function
of scaikring
43+
31+
29+
27+
6.9 12 14 12 13
76 69 58 49 29
9.5 12 17 25 38
7.6 7 11 14 21
l
0.0 0.15 0.32 0.48 0.65 * Angular
angle
resolution
agreement of poor angular sible for the evidenced by
+O.l”,
see Experimental
section.
the relative abundance exists between the two methods. The resolution used in these experiments is at least partly responloss of definition of features in the angular distributions as the CS2 experiments (see below).
Methane The theoretical and experimental curves display a striking degree of similarity (Fig. 4). The inversion of m/z 15 and 14 at 0.6” parallels the crossing seen at 7 eV in the breakdown curve. In fact, crossing points show the expected behavior in all the systems studied and provide a particularly sensitive means of characterizing the data. Ethanol Ethanol scattered on nitrogen collision gas was also investigated. There is agreement between the theoretical and angle-resolved collision-induced dissociation data. Because of the similarities between these data and those for the other two alcohols studied, the results are simply given in Table 2. Carbon
disulfide
Carbon disulfide was chosen to investigate the effect of possible isolated electronic states on angle-resolved CID processes. A low-energy isolated electronic state has been proposed by Hcnglein 1203; this could be the A%, state which lies 2.6 eV above the ground state [ 211. The angle-resolved dissociations (Fig. 5) show a complex behavior, the S” and CS” channels altering in relative importance as the energy is increased. Fragmentation from a single state wouid not be expected to show this behavior and our data suggest that dissociation is occurring independently from several states of CS;-. Figure 5(b) was obtained with increased angular resolution in both the x-y and XYZ planes. The structure evident in the low-resolution spectrum is
238
I . .-
.;;
or*-
67
-o,8
-
T-
3
+_-
-
13
Fig. 5. angular
_.-. 02
cl
“li,-
-
‘--ox
-
,.;
-
-
OG
Ideareel
&NGLE
Angle-rcsolvcd collision-induced dissociations of C’S+’ 2 taken at low resolution. Several CSS’ states appear to bc dissociating independently_
and
higher
clearer in the higher-resolution spectrum. It is also of importance to note that the angle at which CS” begins to dominate S” (ca. 0.05”) and the angle marking the onset of the converse situation, i.e., S” dominating CS” (ca. O-7”), arc identical within experimental error in both the high- and lowresolution spectra. Clearly, the angle-resolved CID measurements are not seriously distorted by instrumental factors. Some results have been obtained on the effectiveness of the target gas in collision-induced dissociation processes. The angle of the cross-over point for the trmsitions, m fz 32’ + 31+ and 32* + 29’ in methanoi, was measured for three target gases: Nz, He, and freon (1,1,2-trichloro-1,2,2-trifhroroethane). These data are tabulated in Table 3. Since the abundances of m/z 29 and 31 are set to be equal, the amount of energy imparted to the projectile ion is the same in each experiment using a different target gas. It is noteworthy
TABLE Target
3 effects
on collision-induced
dissociation
Target
Target
mass (a-m-u.) 28
W? He
c2 C13F3 * In degrees
in x7
plane;
range
where energy
Ol,b/lmI/(ml
ml = mass of the loss upon collision
0lL%il l
+ 712~) -
projectile ion is negligible.
0 cm
15.5
0.49
1.05
24
0.44
3.80
12
0.59
0.66
of angles
yielding an expected angular resolution l * Calculated using the equation 0 czl = arc tan {sin
Target IP (eV)
l
*
Wiis f0.03O (0.5 mm angle-resolving slit width) reproducibility in Olub is %O%_
of +O.OS”;
cos e,abj} and
rnz = mass
of
target;
it is assumed
that
the
239
that the laboratory scattering angles are very similar for the three targets in spite of the large range in their masses. Helium is the most effective target in terms of degree of internal excitation provided for a given laboratory scattering angle; on the other hand, it is ineffective in scattering projectiles through large angles and this is reflected in the O,, data of Table 3. It is important to distinguish between (i) the magnitude of the energy transferred in collisions leading to ionic products which is measured here, and (ii) the probability that a given collision will result in ionic as opposed to neutral projectiles (formed by charge exchange). Both factors govern the effectiveness of a target gas in collision-induced dissociation 1221, but the effects of the latter are not followed in these experiments. Moreover, the effectiveness of the target is not a function of mass -alone and further experiments are in progress to delineate the target gas parameters. CONCLUSION
The results clearly show that selection of the internal energies of polyatomic ions can be achieved by observing collision-induced dissociations as a function of scattering angle. The remarkably good correspondence between these data and the breakdown curves shcwn in Figs. 1-S is particularly apparent when the crossing points for different reactions are compared with reference ^,o internal energy (breakdown curves) and scattering angle (these data). The major difference between the breakdown curves and the angleresolved data is that the features in the latter arc broadened by poorer resolution. While higher angular resolution can be achieved, it must be recognized that high resolution in internal energy is not to be expected in this technique. This is in part because identical impact parameters and scattering angles can correspond to different orientations of the polyatomic ion with the target and hence to different degrees of internal excitation. A further loss of resolution will be associated with the kinetic energy release accompanying fragmentation. In spite of these limitations the procedure is readily implemented and the established value [231 of even cruder methods of internal energy selection attest to its potential usefulness. The present findings open up several lines of enquiry. Firstly, this method should allow approximate breakdown curves to be determined; this information is currently difficult to obtain and available for relatively few polyatomic ions. Implementation of this procedure to obtain quantitative breakdown data requires a knowledge of the relationship between scattering angle and internal energy deposited. These quantities correlate remarkably well in the data taken so far [24]; however, many more data are needed on this point. The present procedure may ultimately prove to provide only qualitative data but it does have the advantage of being applicable to ions which do not have stable counterparts as neutra1 molecules, and it is more rapid than the charge exchange method [I] and does not suffer from the limitation that
240
points can only be taken at a limited number of discrete energies corresponding to the recombination energies of the charge-exchange reagent ion. Secondly, the phenomenon of fragmentation from isolated electronic states should be recognizable from the angle-resolved data. The occurrence of such processes in organic ions has been the subject of continued debate 1251. Thirdly, organic ions of relatively high internal energy become accessible by angle selection. Such fragmentations can be expected to be more directly related to molecular structure than those of lower energy ions which have more propensity for rearrangement 1261. General methods of forming highly-excited organic ions have not been available in the past. Fourthly, enquiry into the mechanism of collision-induced dissociation is facilitated. Comparisons between collisional activation and other methods of electronic excitation (for example, charge exchange and electron impact) can be expected to reveal any effects of angular momentum on dissociation pathways. Reactions in which rotational barriers have been implicated [27-301 should receive attention in the future. The present results are consistent with collisional activation by electronic excitation [ 163 _ ACKNOWLEDGEMENT
This work was supported by the National Science Foundation (CHE 7701295 and 76-06142). REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
E. Lindholm, Adv. Chem. Ser., 58 (1966) 1. B. Andlauer and Ch. Ottinger, J. Chcm. Phys., 55 (1971) 1471. T. Baer, AS. Werner, BP. Tsai and SF. Lin, J. Chem. Phys., 61 (1974) 5468. B. Brehm. J.H.D. Eland, R. Frey and A. Kustler, Int. J. Mass Spectrom. Ion Phys., 12 (1973) 197. D.H. Williams and 1. Howe, Principles of Organic Mass Spectromctry, McGraw Hill, London/New York, 1972. 1. Howe and F.W. McLafferty, J. Am. Chem. Sot., 93 (1971) 99. P.J. Derrick, in A. Maccoli (Ed.), International Review of Science in Mass Spectrometry, Vol. 5, Butterworths, London, 1975, p. 1. J.H. Beynon, M. Bertrand, E.G. Jones and R.G. Cooks, J. Chem. Sot., Chem. Commun., (1972) 34 1. C.E. Klotz, D. Mintz and T. Baer, J. Chem. Phys., 66 (1977) 5100. I. Howe and F.W. McLafferty, J. Am. Chem. Sot., 93 (1971) 99. R.C. Dunbar, J. Am. Chem. Sot., 95 (1973) 472. D.J. Underwood and J.Ii. Bowie, J. Chem. Sot., Perkin Trans. 2, (1977) 1670. F.W. McLafferty, R.A. Kornfeld, S.-C. Tsai, 1. Howe and P.F. Bente, J. Am. Chem. sot., 95 (1973) 2120. R.G. Cooks, L. Hendricks and J.H. Beynon, Org. Mass Spectrom., 10 (l975) 625. ‘I’. Wachs and F.W. McLafferty, Int. J. Mass Spectrom. Ion Phys., 23 (1977) 243. J. Durup, in K. Ogata and T. Hayakawa (Eds.), Recent Development; in Mass Spcctrometry, University Park Press, Baltimore, MD, 1970, p. 921.
2.41 17 J.A. Laramee, J.J. Carmody and R.G. Cooks, Int. J. Mass Spectrom. Ion Phys., 31 (1979) 333. 18 M. Vestal and G. Lerner, Fundamental Studies Relating to the Radiation Chemistry of Small Organic Molecules, Office of Aerospace Research USAF, ARC67-01114, 1967. 19 P. Wilmenius and E. Lindholm, Ark. Fys., 21 (1962) 97. 20 A. Henglein, Z. Naturforsch., Teil A, 17 (1962) 37. 21 R. Frey, B. Gotchev, W.B. Peatman, H. Pollak and E.W. Schlag, Int. J. Mass Spectrom. Ion Phys., 26 (1978) 137. 22 MS. Kim and F.W. McLnffcrty, J. Am. Chem. Sot., 100 (1978) 3279. 23 K. .Levscn, Fundamental Aspects of Organic Mass Spcctrometry, Verlag Chemie, WeinheimJNew York, 1978, Chap. 3. 24 J.A. Laramde, P.H. Hcmberger and R.G. Cooks, J. Am. Chem. SOC., in press. 25 LG. Simm, C.J. Danby and J.H.D. Eland, Int. J. Mass Spectrom. Ion Phys.. 1.4 (1974) 285. 26 D.H. Williams and R.G. Cooks, Chem. Commun., (1968) 663. 27 V. Franchetti, B.S. Freiscr and R.G. Cooks, Org. Mass Spectrom.. 13 (1978) 107. 28 d. Los and T.R. Govers, in R.G. Cooks (Ed.), Collision Spectroscopy, Plenum, New York, 1978, Chap. 6. 29 G.R. Hanson, J. Chem. Phys., 62 (1975) 1161. 30 J.P. Flamme, H. Wankenne, R. Locht, J. Momigny, P.J.C.M. Nowak and J. Los, Chcm. Phys., 27 (1976) 45.
ION INTERNAL ENERGY MASS SPECTROMETRY
J.A.
LARAMEE,
Department (Received
P.H.
HEMBERGER
of Chemistry, 5 July
SELECTION
and R.G.
Purdue University,
THROUGH
231
ANGLE-RESOLVED
COOKS
West Lafayette,
IN 47907
(U.S.A.)
1979)
ABSTRACT Collision-induced dissociation of kilovolt energy ions gives product ions whose nature and abundance depend on the scattering angle. Increasing angle corresponds to depositing increased internal energies. The angle-resolved data for methanol, ethanol, propsn-l-01, methane, and propane closely parallel the breakdown curves for these molecular ions. Fragmentations from isolated electronic states are suggested by the angle-resolved data for CS’,-. The results provide further evidence that collisional activation at kilo-electron volt energies occurs by direct 4ectronic excitation. This methodology is also shown to allow the effectivene.ss of different targets in collisional activation to be determined.
INTRODUCTION
Control over ion internal energy is desirable in many mass-spectral studies, particularly in kinetics, but is relatively difficult to achieve. The classical procedure is charge exchange with a rare gas ion [1,27. More recently, photoelectron/photoion coincidence spectroscopy has emerged as a powerful procedure for studying ions with well-defined internal energies [ 3,4]. The selection of ions with varying internal energies has been important in studies on the dissociations of organic ions. Thus rather crude energy selection has been obtained, for example, by comparing dissociations occurring in different regions of the mass spectrometer [ 51 or by comparing spontaneous and collision-induced dissociations [ 61. Field ionization kinetics has extended the accessible range of lifetimes and hence of internal energies ]73_ A number of physical properties vary with ion internal energy, among them kinetic energy release [S-lo], isotope effects 111,121 and branching ratios of competitive fragmentation channels [13]_ Each of these properties is useful in characterizing ion structure. In this paper we explore the possibility that approximate selection of internal energy can be achieved by definition of the scattering angle in an ion/neutral target collision. In all cases the dissociations which ensue from a high energy (keV range) collision are followed as a function of the scattering angle of the product ion. If a correspondence can be assumed between the
232
impact parameter for a collision and the internal energy deposited - hard collisions depositing more energy than soft collisions - then selection of the scattering angle should represent a selection of ion internal energy. This hypothesis has been tested by measuring the collision-induced dissociations (CID) undergone by particular ions as a function of scattering angle. The relative product ion abundances measured as a function of angle arc compared to relative ion abundances as a function of internal energy, viz. to the breakdown curves for the ions in question. It has previously been shown 114,151 that internal energies well in excess of 10 cV can be deposited in organic ions in kilovolt energy collisions at glancing angles. It has also been shown by exact measurements of product ion kinetic energies that the energy loss in the collision (approximately the cndothermicity of the reaction for fast ions, thermal targets and near-zero scattering angles) correlates with the activation energy for the process in question. In particular, for a series of alcohol molecular ions the energy loss associated with CID is approximately equal to that internal energy known from breakdown curves to maximize the generation of the analyzed product ion 1141. The alcohols thus seem to behave on high energy collision analogously to their behavior on electron impact. This is consistent with the view that electronic excitations are dominant in both processes 1161 and led us to select these species for the present study, in which collisional activation is probed further. EXPERIMENTAL
A Hitachi RMH-2 mass spectrometer modified for angle distribution studies on ion/molecule reactions occurring in the second field-free region was used 1173. Operating conditions were 8-kV accelerating voltage, an electron ionizing energy of 70 eV, and an electron current of 1 mA. Accelerating voltage and lens potentials were adjusted in order to position the reactant beam symmetrically in the collision chamber. Either argon or nitrogen collision gas was used. The collision gas pressure was chosen to optimize the signal strength of the collision-induced dissociation of interest. In all cases the gas pressure measured in the first field-free region using an ion gauge was l-2 X 10S6 torr, corresponding to approximately 10 mtorr in the collision chamber. The energy-resolving slit width was 0.31 mm and the a-slit width was adjusted to attenuate the beam by a factor of 2. These conditions gave an angular spread (in the plane of analysis) of the unreacted primary ion beam in the absence of target gas of +0.07” at half maximum with 98% of the ions being transmitted in this range. The angle-resolving slit width was set at 1.25 mm so that the range of scattering angles about the central ray selected for transmission in the plane of analysis . (x-y plane) was usually iO.08” and in the XYZ plane it was *0.25”. Convolution of the experimental beam divergence (+0.07”) with the geometrically determined range of scattering angles (+0.08” ) yields an angular resolution
233 TABLE
1
Estimated precision of measurements Scattering angIe (“) *
Precision (o/o)
0.1 0.5 1
+ 0.5 f2 i10
* Angle measured in the x--y plane.
of t0.1" (x-y plane). Where indicated, higher resolutions were employed; in one experiment the angle-resolving slit was 0.63 mm giving iO.04” scattering angle in the xy pl*anc and the collector slit height was decreased from 10 mm to 2 mm, corresponding to selection of X-Z angles of ;-0.05”. This improved resolution had the expected effect of sharpening the features seen in the lower resolution spectra (compare Fig. 5). A significant deterioration in signal-to-noise ratio occurred as the scattering angle was increased. The totaI CID product abundance at 1” (laboratory) was typically 1% of the value at O”. This deterioration is primarily due to the decreased cross-section for scattering through larger angles. The variation in collection efficiency is not thought to be significant, especially since the position of the magnet was adjusted to optimize transmission of scattered products. The relative abundance precision versus angle is estimated in Table 1. This table applies to all angle-rcsolvcd vaIues presented except for the high-resolution CSz data. Collision-induced dissociations were assigned on the basis of the momentum of the product ions which correspond approximately to an apparent mass of n&n, for the transition rn: -s rnz + m+ The exact momentum is 3 little less than this value owing to cncrgy loss in the collision. At larger angles both the inelastic and the elastic energy loss components increase so that a slight decrease in magnet field strength was needed to keep 3 particular process in focus as the scattering angle was increased. This factor limited the usefulness of a method of taking data which simply involved scanning the angle-resolving slit while detecting the products of a particular reaction. A slower procedure in which complete momentum (mass) spectra were taken at selected scattering angles was therefore generally used. RESULTS AND DISCUSSION The angle-resolved CID spectra of various molecular ions have been obtained using laboratory scattering angles of O-l” and an angular resoluiion of i-0.1” as described above. The relative intensity of the CID transitions as a function of the scattering angle is displayed in Figs. L-5 for some of the systems investigated. Shown for comptiison arc the theoretical breakdown
234
curves calculated [ 181 by quasi-equilibrium theory (QET) and in one case (Fig. 2) the experimental charge exchange data [ 191. Agreement of the overall features of both sets of curves is excellent. This establishes that, at least qualitatively, ion internal energy can be selected by varying the scattcring angle. The size of the data points in the figures is not indicative of the precision of measurement, which decreases with increasing scattering angle. (Signal strengths decreased by a factor of 100 on reaching the higher angle settings.) Where necessary, corrections were made for the contribution of unimolecular dissociation (metastable ions) to the collision-induced dissociation process. This was accomplished by measuring the intensity of the signal of the metastable ion at various angles with no collision gas in the second fieldfree region. In comparing the breakdown curves with the angle-resolved spectra, note that the minimum energy accessible in the latter corresponds to the onset of collision-induced dissociation. Truncating the breakdown curves at a non-zero energy (broken lines) therefore provides the best comparison of the two sets of data. The experimental results for each of the molecular ions may now be discussed in turn. Propan-I-ol (m/z 59) is the most facile process The dissociation of M” to (M -H)* sampled at zero-scattering angle. The rapid depletion of this ion with increasing scattering angle is associated with the opening of several reaction chanI - PROPANOL
ENERGY
bV)
.
k?
COLLISION
GAS
ANGLE
(d.Qrsd
Fig. 1. Comparison of the theoretical breakdown curve of the molecular ion of propanl-01 [lS] with the collision-induced dissociations of this ion recorded as a function of laboratory scattering angle. All data are given as relative abundance. The dashed line indicates the position on the breakdown curve corresponding to the zero-angle CID results. At lower energies the molecular ion dominates and there is no parallel in the angleresolved data.
236
METHANOL.
COLLISION
GAS
OET CALCULATION
60t!! 3
Ng
=O
Cl0
(\
2 Q IiJ
40-
= !i 2 az
20-
0
2
4
6
ENERGY
]
m’
cl2
04
06
ANGLE
08 kJegrss)
23
I--._.
I2
0
CHARG~I EXCHANGE
k IO
6 (avl
14
ENERGY
3!?.../
I6
IS
(SW
Fig. 2. Breakdown curve 1161 for the methanol reso!vcd CII3 and charge exchange data [ 17 1.
molecular
ion
compared
with
angle-
nels for fragmentation of (M - H)‘. The theoretical breakdown curve shows ions 59’ and 29*, (C,H,)‘, to be of equal abundance at the maximum for 31’, (CH,OH’) (Fig. 1). This featur,p is reproduced in the angle-resolved CID curve. Agreement of the two curves is preserved at high internal energy and large scattering angle. (Data for 27’ are not given in ref. 18.) Me thanoi Many of the features of propan-l-01 are preserved in the methanol plot (Fig. 2), for example, the rapid decrease of (M - H)’ and the increasing rela-
236
1 90
PROPANE,
-
N2
toU.lSlON
GAS
29
4
2
0
9
6 ENERGY
Fig. 3. Breakdown curve [16] resolved CID data.
a9
Cl6
04
0.2
0
IO
ANGLE
fcV)
(degree)
for the propane molecular ion compared with the angie-
tivc abundance of 29’ at high angle. The elimination of Hz from the methanol molecular ion is a minor process at any angle. The agreement among the breakdown curve, the charge exchange data, and the angular-resolved curves is good. It is particularly noteworthy that the two experimental procedures give closely corresponding results. Propane Although the changes in relative abundance of the ions in the CID curve (Fig. 3) are not nearly as sharp as those predicted by QET, good qualitative
\:
METHANE.
>
15
90.
60
-
40
-
COLUSION
GAS
.--_ ,_
-‘-A. _/._____.
\
I5
>
20
14
1
Al
__--
‘\.___.....
14
.,-‘.
~
i
02’ ENLRGY
Fig. 4. Breakdown curve resolved di.ssociations.
[ 16 ] for the mothane
molecular
08
OG
04 ANGLE
WI
10
(degree)
ion compared
to the angle-
237 TAl3LE
2
Collision-induced Laboratory angle (“)
dissociation
of C2HSOH+-
as a function
of scaikring
43+
31+
29+
27+
6.9 12 14 12 13
76 69 58 49 29
9.5 12 17 25 38
7.6 7 11 14 21
l
0.0 0.15 0.32 0.48 0.65 * Angular
angle
resolution
agreement of poor angular sible for the evidenced by
+O.l”,
see Experimental
section.
the relative abundance exists between the two methods. The resolution used in these experiments is at least partly responloss of definition of features in the angular distributions as the CS2 experiments (see below).
Methane The theoretical and experimental curves display a striking degree of similarity (Fig. 4). The inversion of m/z 15 and 14 at 0.6” parallels the crossing seen at 7 eV in the breakdown curve. In fact, crossing points show the expected behavior in all the systems studied and provide a particularly sensitive means of characterizing the data. Ethanol Ethanol scattered on nitrogen collision gas was also investigated. There is agreement between the theoretical and angle-resolved collision-induced dissociation data. Because of the similarities between these data and those for the other two alcohols studied, the results are simply given in Table 2. Carbon
disulfide
Carbon disulfide was chosen to investigate the effect of possible isolated electronic states on angle-resolved CID processes. A low-energy isolated electronic state has been proposed by Hcnglein 1203; this could be the A%, state which lies 2.6 eV above the ground state [ 211. The angle-resolved dissociations (Fig. 5) show a complex behavior, the S” and CS” channels altering in relative importance as the energy is increased. Fragmentation from a single state wouid not be expected to show this behavior and our data suggest that dissociation is occurring independently from several states of CS;-. Figure 5(b) was obtained with increased angular resolution in both the x-y and XYZ planes. The structure evident in the low-resolution spectrum is
238
I . .-
.;;
or*-
67
-o,8
-
T-
3
+_-
-
13
Fig. 5. angular
_.-. 02
cl
“li,-
-
‘--ox
-
,.;
-
-
OG
Ideareel
&NGLE
Angle-rcsolvcd collision-induced dissociations of C’S+’ 2 taken at low resolution. Several CSS’ states appear to bc dissociating independently_
and
higher
clearer in the higher-resolution spectrum. It is also of importance to note that the angle at which CS” begins to dominate S” (ca. 0.05”) and the angle marking the onset of the converse situation, i.e., S” dominating CS” (ca. O-7”), arc identical within experimental error in both the high- and lowresolution spectra. Clearly, the angle-resolved CID measurements are not seriously distorted by instrumental factors. Some results have been obtained on the effectiveness of the target gas in collision-induced dissociation processes. The angle of the cross-over point for the trmsitions, m fz 32’ + 31+ and 32* + 29’ in methanoi, was measured for three target gases: Nz, He, and freon (1,1,2-trichloro-1,2,2-trifhroroethane). These data are tabulated in Table 3. Since the abundances of m/z 29 and 31 are set to be equal, the amount of energy imparted to the projectile ion is the same in each experiment using a different target gas. It is noteworthy
TABLE Target
3 effects
on collision-induced
dissociation
Target
Target
mass (a-m-u.) 28
W? He
c2 C13F3 * In degrees
in x7
plane;
range
where energy
Ol,b/lmI/(ml
ml = mass of the loss upon collision
0lL%il l
+ 712~) -
projectile ion is negligible.
0 cm
15.5
0.49
1.05
24
0.44
3.80
12
0.59
0.66
of angles
yielding an expected angular resolution l * Calculated using the equation 0 czl = arc tan {sin
Target IP (eV)
l
*
Wiis f0.03O (0.5 mm angle-resolving slit width) reproducibility in Olub is %O%_
of +O.OS”;
cos e,abj} and
rnz = mass
of
target;
it is assumed
that
the
239
that the laboratory scattering angles are very similar for the three targets in spite of the large range in their masses. Helium is the most effective target in terms of degree of internal excitation provided for a given laboratory scattering angle; on the other hand, it is ineffective in scattering projectiles through large angles and this is reflected in the O,, data of Table 3. It is important to distinguish between (i) the magnitude of the energy transferred in collisions leading to ionic products which is measured here, and (ii) the probability that a given collision will result in ionic as opposed to neutral projectiles (formed by charge exchange). Both factors govern the effectiveness of a target gas in collision-induced dissociation 1221, but the effects of the latter are not followed in these experiments. Moreover, the effectiveness of the target is not a function of mass -alone and further experiments are in progress to delineate the target gas parameters. CONCLUSION
The results clearly show that selection of the internal energies of polyatomic ions can be achieved by observing collision-induced dissociations as a function of scattering angle. The remarkably good correspondence between these data and the breakdown curves shcwn in Figs. 1-S is particularly apparent when the crossing points for different reactions are compared with reference ^,o internal energy (breakdown curves) and scattering angle (these data). The major difference between the breakdown curves and the angleresolved data is that the features in the latter arc broadened by poorer resolution. While higher angular resolution can be achieved, it must be recognized that high resolution in internal energy is not to be expected in this technique. This is in part because identical impact parameters and scattering angles can correspond to different orientations of the polyatomic ion with the target and hence to different degrees of internal excitation. A further loss of resolution will be associated with the kinetic energy release accompanying fragmentation. In spite of these limitations the procedure is readily implemented and the established value [231 of even cruder methods of internal energy selection attest to its potential usefulness. The present findings open up several lines of enquiry. Firstly, this method should allow approximate breakdown curves to be determined; this information is currently difficult to obtain and available for relatively few polyatomic ions. Implementation of this procedure to obtain quantitative breakdown data requires a knowledge of the relationship between scattering angle and internal energy deposited. These quantities correlate remarkably well in the data taken so far [24]; however, many more data are needed on this point. The present procedure may ultimately prove to provide only qualitative data but it does have the advantage of being applicable to ions which do not have stable counterparts as neutra1 molecules, and it is more rapid than the charge exchange method [I] and does not suffer from the limitation that
240
points can only be taken at a limited number of discrete energies corresponding to the recombination energies of the charge-exchange reagent ion. Secondly, the phenomenon of fragmentation from isolated electronic states should be recognizable from the angle-resolved data. The occurrence of such processes in organic ions has been the subject of continued debate 1251. Thirdly, organic ions of relatively high internal energy become accessible by angle selection. Such fragmentations can be expected to be more directly related to molecular structure than those of lower energy ions which have more propensity for rearrangement 1261. General methods of forming highly-excited organic ions have not been available in the past. Fourthly, enquiry into the mechanism of collision-induced dissociation is facilitated. Comparisons between collisional activation and other methods of electronic excitation (for example, charge exchange and electron impact) can be expected to reveal any effects of angular momentum on dissociation pathways. Reactions in which rotational barriers have been implicated [27-301 should receive attention in the future. The present results are consistent with collisional activation by electronic excitation [ 163 _ ACKNOWLEDGEMENT
This work was supported by the National Science Foundation (CHE 7701295 and 76-06142). REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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