Ionization of Doped Helium Nanodroplets - American Chemical Society

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Ionization of Doped Helium Nanodroplets: Residual Helium Attached to Diatomic Cations and Their Clusters Benjamin Shepperson, Jin Liu, Andrew M. Ellis,* and Shengfu Yang* Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom ABSTRACT: Electron impact ionization of helium nanodroplets containing a dopant, M, can lead to the detection of both Mþ and helium-solvated cations of the type Mþ 3 Hen in the gas phase. The observation of helium-doped ions, HenMþ, has the potential to provide information on the aftermath of the charge transfer process that leads to ion production from the helium droplet. Here we report on helium attachment to the ions from four common diatomic dopants, M = N2, O2, CO, and NO. For experiments carried out with droplets with an average size of 7500 helium atoms, the monomer cations show little tendency to attach and retain helium atoms on their journey out of the droplet. By way of contrast, the corresponding cluster cations, Mnþ, where n g 2, all show a clear affinity for helium and form HemMnþ cluster ions. The stark difference between the monomer and cluster ions is attributed to more effective cooling of the latter in the aftermath of the ionization event.

’ INTRODUCTION It is now almost two decades since molecule-doped helium nanodroplets became the target of frequent laboratory-based studies.1 Although optical spectroscopy provides the greatest detail on molecular behavior in helium nanodroplets, electron impact (EI) ionization mass spectrometry is still commonly used to probe these systems, whether as a simple diagnostic or for a more detailed interrogation of the droplet contents. The commonly accepted model of molecule ionization in helium nanodroplets involves an indirect resonant charge transfer process.2 In droplets of any significant size, the most likely initial ionization event will be the removal of an electron from a helium atom near the surface of the droplet. Because all atomic and molecular dopants contained within the droplet will possess a lower ionization energy than that of atomic helium, there is a thermodynamic tendency for the positive charge to ultimately reside on the dopant. This can be achieved by resonant charge hopping, which can deliver the positive charge onto the dopant, provided the journey requires no more than roughly 10 hops.3,4 The earliest mass spectrometric studies of doped helium droplets derived from work by Toennies and his colleagues, who investigated dopants such as SF6,5 Ar, Kr, Xe, and H2O.6 One of the most notable observations was for SF6, where ionization led to SF5þ as the major product. Although ion fragmentation is not avoided, it is clearly affected by the helium because there is almost no production of smaller fragments, such as SF3þ, which contrasts sharply with gas phase experiments. Subsequently, Janda and co-workers investigated electron impact ionization of both rare gas atoms and NO molecules in helium droplets.710 For NO-doped helium droplets, (NO)nþ clusters (n e 4) were the dominant products. A dramatic decrease in charge transfer probability with an increase of helium droplet size was reported, and the charge transfer probability was found to be r 2011 American Chemical Society

slightly higher for NO clusters than that of NO monomer.10 In recent work, Lewis and co-workers carried out a detailed study of the fragmentation of the triphenylmethanol cation in helium droplets and a significant change in the fragmentation pattern from the helium droplet case was observed when compared with the corresponding gas phase EI mass spectrum.3 Even more recently, in our laboratory we have systematically investigated ion fragmentation for haloalkanes,11 alcohols,12 and their clusters inside helium droplets.13 We found that helium droplets can alter the fragmentation patterns of the embedded molecules and clusters in comparison to gas-phase mass spectrometry. However, only in very few cases, for example, cyclopentanol and cyclohexanol, was soft ionization particularly evident. In other words, while helium droplets can dissipate excess energy through evaporative loss of helium atoms, the rate of cooling for many molecular ions is insufficient to prevent copious fragmentation. In addition to the production of bare ions, ions are sometimes detected with one or more helium atoms attached. Most commonly this is seen for atomic dopants, for example, Ar,7 Ne,8 Xe,9 or Li.14 In contrast, molecular ions show less tendency to emerge from the droplets with one or more helium atoms in tow. The reason put forward for this is that residual internal energy in the departing molecular ions, in the form of vibrational and rotational energy, leads to complete evaporative loss of helium atoms.15 However, while a useful rule of thumb, it is not universally true and several examples of molecular cations being detected with attached helium atoms have been reported in the Special Issue: J. Peter Toennies Festschrift Received: December 23, 2010 Revised: April 26, 2011 Published: May 18, 2011 7010

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The Journal of Physical Chemistry A literature. An early example was reported by Callicoatt et al.,10 who observed NOþ and (NO)2þ ions with up to 15 attached helium atoms. More recently, F
arník and Toennies mixed together two reagents in a single helium nanodroplet and initiated ionmolecule reactions via electron impact ionization, including N2þ þ D2, CH4þ þ D2, and CH3þ þ D2. Molecular reaction intermediates with a few helium atoms attached were observed, such as HemCH3þ (m = 1, 2, 4) and HemCH4þ (m = 1, 2).15 The observation of ions emanating from helium nanodroplets with or without attached helium atoms may be considered little more than a curiosity. However, there is the potential to use these observations to extract information about the events occurring subsequent to charge transfer onto the dopant. The process by which ions are produced in helium nanodroplets and arrive in the gas phase is currently very poorly understood, so such information would be valuable. Because there have been no systematic studies of helium attachment to molecular ions, we report the first such investigation here, focusing on four common diatomic dopants, N2, O2, CO, and NO, together with their corresponding molecular clusters. A sharp distinction is found between the diatomic monomers and the clusters, with cations from only the latter showing any significant tendency to emerge with helium atoms attached. Several possible explanations are explored but we conclude that the key reason for these observations is that the cluster ions are able to attain much lower temperatures than the monomer cations as they depart into the gas phase.

’ EXPERIMENTAL SECTION The experimental apparatus has been fully described previously.16 Briefly, the apparatus consists of a pulsed helium droplet source, a pick-up cell for addition of dopants, and a reflectron time-of-flight mass spectrometer equipped with an electron impact ionization source (∼100 eV electron impact energy). The pulsed droplet source consisted of a solenoid valve (General Valve series 99) with a Kel-F poppet and a homemade faceplate, which was cooled by a closed-cycle cryostat. For all the experiments in this work, the helium stagnation pressure was fixed at 20 bar, and the nozzle temperature was 15 K, giving helium droplets of average size ÆNæ ≈ 7500.16 Our aim in using a fixed droplet size was to focus attention on the differences between dopants under constant cooling conditions. The quantity of added dopant gas was controlled by a fine leak valve and was generally set such that the monomer and small clusters were the principal constituents of the helium droplets. Computational Details. As an aid to interpretation of the experimental findings, ab initio calculations were carried out on He(N2)xþ, He(CO)xþ, He(O2)xþ, and He(NO)xþ (x = 1, 2). There have been relatively few calculations carried out previously on these ionic clusters. The first and only prior detailed study of HeN2þ was reported by Miller et al., who used MCSCF-CI methodology.17 The calculations were carried out at a fixed NN bond length and yielded a binding energy of 140 cm1 for the helium atom and a very weak dependence of this binding energy on the HeNN bond angle. The earliest calculations on HeCOþ were reported by Hamilton et al.,18 but these considered only three possible orientations of the COþ relative to the helium atom. A more complete study was reported by Maclagan et al.,19 which employed the MP4SDTQ method in combination with a 6-311þþG(3df,3pd) basis set. The global potential energy minimum was found to occur when the He atom binds at the C end of COþ with a HeCO bond angle of 43.6. The potential

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Figure 1. Mass spectra of NO picked up by helium droplets at (a) relatively high NO partial pressure and (b) at low NO partial pressure. In panel (a), a background mass spectrum with no added dopant has been subtracted to remove the majority of Henþ cluster ion signals. Note the abrupt change of vertical scale at low m/z beyond NOþ in spectrum (b).

energy well binding the helium atom was found to be 298 cm1 deep at the minimum energy structure. HeNOþ has been more extensively studied by ab initio methodology. Early studies included those by Robbe et al.,20 Pogrebyna et al.,21 and Viehland et al.22 However, the most comprehensive studies have been reported by Lee and co-workers, and this team has reported a very high level calculation of the ground electronic state potential energy surface using the CCSD(T) method with large basis sets up to, and including, aug-ccpV5Z.2325 The HeNOþ system was found to be linear at the global potential energy minimum and a basis set extrapolation procedure gave a best estimate of 198 ( 4 cm1 for the depth of the potential energy well which binds the helium atom. No ab inito calculations have been reported for HeO2þ, nor have any ab initio calculations been reported for helium bound to any of the diatomic dimer cations studied in the current work, that is, (N2)2þ, (CO)2þ, (O2)2þ, and (NO)2þ. Because a comparison of the binding energy of helium to the monomer and dimer cations is important in the present study, the necessary calculations are reported here. These calculations were carried out at the RCCSD(T) level using the aug-cc-pVTZ basis sets for all of the atoms.26,27 Corrections for the basis set superposition error were incorporated using the counterpoise procedure and the calculations were performed using MOLPRO.28

’ RESULTS 1. Mass Spectrometry. Callicoat et al. have previously reported that both NOþ and (NO)2þ can retain some helium atoms when produced from helium droplets following electron impact ionization.10 Consequently, we first present our results for NO-doped helium droplets for comparison with those of Callicoat et al. before turning to the other diatomic dopants. Figure 1 shows a portion of the mass spectrum centered on the dimer region. Because the mass of NOþ does not clash with the mass of any helium cluster ions, Hemþ, the observation of HemNOþ clusters is unambiguous. The same is not true for (NO)2þ, but the effect of any contribution from Hemþ ions can largely be removed by subtraction of a background mass 7011

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Figure 2. Portion of the mass spectrum from O2 clusters in helium nanodroplets. This spectrum was obtained after subtraction of a background spectrum with no added dopant. Extra peaks are observed due to residual N2 and H2O in the background: Δ represents H2O 3 Onþ, and x identifies N2O2þ (which coincides with He3O3þ).

spectrum in which no NO was added. This gives the spectrum shown in Figure 1a, in which the relatively strong peaks attributed to Hem(NO)2þ arise almost entirely from these cluster species. It seems from inspection of Figure 1a that both the monomer and the dimer cations have comparable propensities for attaching helium atoms. However, when the partial pressure of NO added to the pickup cell is decreased, such that a single NO in the helium droplets is the dominant outcome, the HemNOþ signal becomes exceptionally weak, as can be seen in Figure 1b. At the highest doping level used in this work, the HeNOþ/NOþ signal ratio was about 8% (Figure 1a), while at very low doping pressures, where the (NO)2þ peak is extremely weak in the mass spectrum, the ratio fell to 0.25% (Figure 1b). This finding broadly agrees with that reported previously by Calicoatt et al. and is consistent with their explanation, namely, that the majority of HemNOþ ions seen in Figure 1a arise from fragmentation of NO clusters inside helium droplets after the ionization event. We now seek to make a comparison with the other diatomic molecules. What we find for these dopants is similar to the NO case, except that the low helium affinity by the monomer cations is even more marked, with almost no detectable quantities of HemMþ being produced under any conditions. However, some dimer and larger cluster ions emerge with attached helium atoms. Figure 2 illustrates this for O2 addition. Notice that when O2 is the dopant, clusters containing both odd and even numbers of oxygen atoms are observed, in line with mass spectrometric studies of molecular oxygen clusters in the gas phase.35 The even clusters are generally more abundant than the odd cluster ions, which is again in agreement with gas phase studies. Also in agreement with prior gas phase studies, O5þ is seen as a magic number peak, that is, it has an anomalously high abundance compared with other odd oxygen cluster ions. The production of odd-numbered cluster ions suggests a highly energetic environment for ion formation, leading to bond dissociation of O2þ, despite the nominally cold liquid helium matrix surrounding the (O2)n clusters prior to ionization. This is consistent with the large amount of energy, in excess of 10 eV, which is expected to be deposited into the ions on charge transfer from Heþ. As will be discussed later in the context of ions with attached helium atoms, the helium is not efficient at removing such large quantities of energy.

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Figure 3. Mass spectrum from N2 clusters doped in helium nanodroplets after subtraction of a background spectrum with no added N2. Δ represents (N2)n 3 H2Oþ and x identifies binary clusters of (N2)n 3 O2þ.

Despite a large O2þ peak in the mass spectrum, there is no evidence of any significant formation of HemO2þ ions. On the other hand, all larger clusters, both odd and even varieties, show a series of Hem(On)þ ions with gradually decreasing intensities. The largest cluster ion seen in the current work with a significant signal/noise ratio is O22þ, which still shows some helium attachment. A portion of the N2 mass spectrum is shown in Figure 3. Unlike the mass spectrum of O2 clusters, in which odd-numbered cluster ions such as O5þ were seen with significant intensities, the spectra derived from N2 in helium droplets were dominated by even-numbered N2 clusters, (N2)nþ, under the conditions used in the current study. The only observable odd nitrogen species is N3þ, which shows about 3% of the intensity of (N2)2þ. This is broadly in line with the gas-phase observation of ionmolecule reactions involving N2 clusters.36 Similar to the case of O2, helium attachment is almost negligible for N2þ but becomes significant for the dimer and larger cluster ions, and we have seen Hem(N2)nþ ions for clusters as large as n = 11. We note that F
arník and Toennies previously reported the production of very weak HemN2þ clusters on electron impact ionization of nitrogendoped helium droplets, with m restricted to only 1 and 2.15 This difference may arise from the larger helium droplets (ÆNæ = 1.2  104 helium atoms) employed in the study by F
arník and Toennies, which should improve the cooling and thus would assist the survival of HemN2þ clusters. To complete the presentation of the mass spectral data, Figure 4 shows a section from the CO experiments. To assist with the discrimination between Hemþ and Hem(CO)nþ ions, data have been recorded for both naturally abundant CO and for 13 CO. Unfortunately, the spectrum is contaminated by residual H2O and N2, so several extra peaks are present in Figure 4. Nevertheless, when comparing the 12CO and 13CO spectra, it is clear that helium attachment to COþ is negligible, in stark contrast to (CO)nþ cluster ions from the dimer onward. When the 13CO spectrum is used, (CO)2þ ions can be seen with up to 14 attached helium atoms before the signal decays below an observable level. Note that mixed cluster ions of the type N2(CO)nþ also show attached helium atoms. 2. Computational Results. At the RCCSD(T) level of theory, only one potential energy minimum was obtained for each helium-attached monomer ion and, in all four cases, the corresponding structure was nonlinear. The calculated binding 7012

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Figure 5. Optimized geometries for (a) HeN2þ and (b) He(N2)2þ. For HeN2þ we obtain R1 = 1.121 Å, R2 = 2.651 Å, and θ =105.8. He(N2)2þ, which shows C2v symmetry, has R1 = 1.109 Å, R2 = 2.014 Å, R3 = 2.905 Å, and θ = 178.9.

Figure 4. Mass spectra obtained from (a) 13CO and (b) 12CO in helium nanodroplets. In the upper panel, r marks the N213COþ peak and x represents the 13CO2þ peak. Owing to residual H2O in the vacuum chamber, peaks due to CO OHþ, CO H2Oþ, and (CO)2Hþ are also seen.

Table 1. Comparison of Helium Binding Energies for the Diatomic Monomer and Dimer Cations helium binding cluster

energy/cm1a

HeN2þ

160

HeCOþ

207

HeO2þ

173

HeNOþ

186

He(N2)2þ

131

He(CO)2þ

151

He(O2)2þ

128

He(NO)2þ

91/158b

Figure 6. The optimized geometries for (a) HeCOþ and (b) He(CO)2þ (Cs symmetry). For HeCOþ we have R1 = 1.112 Å, R2 = 2.476 Å, and θ = 125.4. For He(CO)2þ, R1 = 1.138 Å, R2 = 1.138 Å, R3 = 1.533 Å, R4 = 2.912 Å, R5 = 2.993 Å, θA = 143.5, and θB = 143.3.

a

These are De values and include a counterpoise correction for the basis set superposition error. b cis/trans values.

energies for helium are summarized in Table 1. We note that our calculated binding energy for HeNOþ, which was found to be 186 cm1, is in good agreement with that obtained by Sold
an et al. at the CCSD(T)/aug-cc-pV5Z level (193 cm1)25 and is also reasonably close to their value of 198 ( 4 cm1 obtained by extrapolation to the complete basis set limit. This illustrates the quality of the current set of calculations. For the diatomic dimer cations, the calculations found mostly planar potential energy minima when helium was attached. For illustration, the structures for stable dimer cations are compared with their monomer analogues in Figures 58. Because the weakly bound helium atom will only slightly perturb the structure of molecular ions in most cases, the optimized structures of the diatomic dimer ions were used as the initial geometries of the dimer ions in the He-attached clusters. (N2)2þ is known to have D¥h symmetry3032 so this was used as the initial structure for (N2)2þ in the helium attached cluster. A geometry optimization of He(N2)2þ confirmed the negligible impact of the helium on the structure of (N2)2þ, which remained essentially linear (see Figure 5). For (CO)2þ, only a trans-[OC 3 þ 3 3 CO] structure was found to be stable in the dissociative

Figure 7. Optimized geometries for (a) HeO2þ and (b) He(O2)2þ. For HeO2þ, R1 1.122 Å, R2 = 2.718 Å, and θ = 78.5. For He(O2)2þ, R1 = 1.164 Å, R2 = 1.164 Å, R3 = 2.153 Å, R4 = 2.815 Å, R5 = 3.264 Å, θA = 112.7, and θB = 114.6.

ionization of Ar/CO clusters by M€ahnert et al.29 Consequently, the trans-[OC 3 3 3 CO]þ was used as the starting structure of (CO)2þ in the geometry optimization of He(CO)2þ in the current work. The (CO)2þ core maintained this structure at the end of the optimization (see Figure 6). For (O2)þ, previous calculations found that the lowest energy spin state is a quartet and 11 distinct stationary points were found on the ground state potential energy surface.34 Because our aim was to show the impact of dimerization on the binding energy of an attached helium atom, we chose only one starting structure for (O2)þ, a trans structure similar to that of (CO)2þ. When helium was added, this structure was maintained, as can be seen in Figure 7. For (NO)2þ, two stable structures were identified by Lee et al. in high quality ab initio calculations, a cis and a trans structure.33 The cis structure lies only about 100 cm1 above the trans 7013

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Figure 8. Optimized geometries for (a) HeNOþ and (b)/(c) He(NO)2þ (derived from both the cis and trans isomers of (NO)2þ). For HeNOþ, R1 = 1.065 Å, R2 = 2.731 Å, and θ = 88.8. For He(NO)2þ, starting from the cis (NO)2þ, we have R1 = 1.110 Å, R2 = 2.252 Å, R3 = 2.945 Å, and θ = 103.4. For He(NO)2þ, starting from the trans (NO)2þ, we have R1 = 1.109 Å, R2 = 2.226 Å, R3 = 2.856 Å, and θ = 118.8.

structure, so both were considered in the current work. For He(NO)2þ both cis and trans structures were found to yield potential energy minima (see Figure 8), with the trans structure providing the global minimum. The calculations on He(NO)2þ, when starting from trans (NO)2þ as the initial structure, gave the only nonplanar structure among the diatomics studied in this work, with the He atom lying out of the (NO)2þ plane. The binding energy of a helium atom to each of the diatomic dimer cations is summarized in Table 1 alongside the monomer binding energies. The crucial observation is that the binding energies are all substantially less for diatomic dimer cations than for the corresponding monomer cations.

’ DISCUSSION The key experimental observation from this work is that diatomic cations are derived from helium droplets with no helium atoms attached, whereas all diatomic cluster cations, irrespective of size, show attached helium atoms. The one caveat concerns NOþ, because HemNOþ ions were detected. However, dopant pressure-dependent experiments clearly show that the HemNOþ ions originate from the dissociation of (NO)nþ cluster ions, that is, there is no significant tendency for NOþ derived from isolated NO to pick up and retain helium atoms through to ion detection. The fact that all cluster cations, Mnþ, where M is the diatomic monomer, show a tendency to attach helium atoms almost irrespective of size is initially surprising. The cluster cations are polyatomic systems and, therefore, according to previous claims,15 should show a lower tendency to retain helium atoms than Mþ, whereas we observe the opposite effect. We must therefore consider the factors that might be responsible for these findings. Three possibilities are considered here: (i) cluster energetics and stabilities, (ii) cluster dissociation rates, and (iii) the cluster temperature.

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Before discussing these specific issues, we point out that we expect all of the diatomic molecules we have considered to submerge within the helium droplet and to form clusters prior to ionization. Although the intermolecular potential energy well is rather shallow for some of the Mn clusters, it still strongly exceeds the HeHe interaction and the Hemolecule interaction. For example, (O2)2, which is by some way the most weakly bound of the neutral diatomic dimers, is calculated to have a binding energy in excess of 80 cm1 in its spin quintet state.37 By comparison, the well depth for He attached to O2 is ∼7 cm1,38 so the thermodynamically favored state will occur when two O2 molecules combine rather than remain as separately solvated molecules in the helium droplet. For the other diatomics, this thermodynamic tendency for cluster formation is even stronger. 1. Cluster Binding Energies. Taking energetics first, we must consider the possibility that the binding of helium atoms to the diatomic cluster ions is stronger than to the diatomic monomer ions. If correct, this could account for the preferential ability of cluster ions to retain helium atoms as they depart from the helium droplets. However, this would be surprising given that, in the cluster cations, there is more opportunity for charge delocalization, which if anything should have the effect of weakening the binding of helium atoms by diminishing the strength of the charge-induced dipole interaction. Our series of ab initio calculations, described earlier, have confirmed this expectation. The binding energies for helium attachment to all of the monomer ions are significantly higher than those for the dimer ions and we can therefore dismiss binding energy, that is, cluster stability, as a determining factor in the survival of ions with attached helium atoms. 2. Rates of Decomposition. Another reason why HemMnþ ions are readily seen, whereas HemMþ ions are not, may derive from differential rates of decomposition. In other words, due to the internal energy content following charge transfer from Heþ, HemMþ ions might undergo more rapid unimolecular decay than HemMnþ ions and thus only the latter survive to detection. This explanation is initially appealing, because on a purely statistical basis, the decay rate for HemMnþ ions should be significantly lower than for HemMþ ions. However, there are two problems with this suggestion. First, it would require that all HemMþ ions fully decay within the period between initial ionization and ion detection, which is on the order of 10 μs, whereas many or all of the HemMnþ ions do not. This would be a remarkable coincidence for all of the ions explored in the current study. In particular, the ionization energies of the diatomics considered in this work span the range from 9.264 eV (NO) to 15.581 eV (N2).39 Consequently, different ions would be formed with dramatically different internal energies when charge transfer from Heþ takes place, and one would therefore anticipate major variations in mass spectral outcomes if unimolecular decay is the sole explanation. However, no such dramatic differences are observed and, thus, we rule out differential ion fragmentation rates as the explanation for HemMnþ survival. 3. Cluster Temperatures. We are therefore left with the possibility that the cluster cations are colder than the monomer cations, thus, making it more likely that the former can bind helium atoms without subsequent evaporative loss. One obvious difference between the diatomic monomer cations, Mþ, and the cluster cations, Mnþ, is that the latter have the option of dissipating any excess energy not only by energy transfer to the surrounding helium, but also by evaporation of monomers (M) 7014

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The Journal of Physical Chemistry A from the cluster. Because the binding energies of the diatomic monomers to the cluster ion are much larger than for helium atoms, monomer evaporation can potentially remove much larger quantities of energy than helium atom evaporation. Thus, when the molecular cluster ions are formed with initially high internal energies following charge transfer from Heþ, they can potentially achieve a much lower temperature than the diatomic monomer cations. Lewis et al. suggested a similar explanation to account for the reduced fragmentation of the parent ions of triphenylmethanol (TPM) when sufficient TPM was added to helium droplets to form clusters.3 However, if monomer evaporation were solely responsible, we would expect to see cold monomer cations as the end product of this evaporative chain and therefore we should see helium atoms attached to these species. This only happens for NOþ, so there must be another factor at work. The additional factor is most likely to be the increased efficiency of energy removal by evaporative loss of helium atoms for cluster ions when compared with the monomer. This increased efficiency derives from the low frequency intermolecular vibrations possessed by the cluster ions, which are able to exchange energy more rapidly with the surrounding helium than do the monomer cations, with their single, relatively high frequency, vibration. Consequently, there are two factors that favor more rapid cooling of the cluster cations over the diatomic monomer cations and which are, therefore, expected to confer a much lower temperature on the former compared with the latter. This picture of enhanced cooling for cluster cations can be extended to include anions. Recent observations of anions produced from doped helium nanodroplets by low energy electron injection has seen a similar preference for helium attachment to dimer and larger cluster anions when compared with monomers. These observations apply to larger molecules than diatomics, such as formamide40 and the amino acids glycine, alanine, and serine.41 It has been suggested previously that the binding energy may be a key factor in helium retention for the dimer and larger clusters; for example, in the case of the amino acids, zwitterionic forms of the anions may be favored in helium droplets, which could enhance interaction with helium atoms through charge-induced dipole interactions.41 However, the present work suggests that enhanced cooling alone through the mechanism described above could explain the preferential helium attachment to such cluster anions. 4. Survival of HemNOþ Cluster Ions. Finally, we comment on the survival of HemNOþ cluster ions. We noted earlier that these ions derive from fragmentation of (NO)nþ (n g 2) cluster ions in the helium droplets, but it is not obvious why the other diatomic dopants do not show analogous behavior. It is particularly surprising given that NO has the lowest first ionization energy of the four molecules considered in this work, and therefore, the internal energy content on charge transfer from Heþ is ostensibly the highest. Thus, on this basis, NOþ would be the least likely of the diatomic monomer cations to successfully attach helium atoms because it would be formed as the hottest ion. Other factors must clearly be at work here and at this stage we can only speculate on the source of the distinct behavior for NO. We have considered issues such as Penning ionization by metastable helium atoms, which can be produced by electron impact on helium droplets,42 but there would be no obviously preferential route here to cold NOþ that would not also be available to the other diatomics. We have also considered the formation of electronically excited cluster ions, either following charge transfer

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from Heþ or by Penning ionization. Such ions could remove large quantities of energy by fluorescence, but such channels are once again open to the other diatomics and there is therefore no obvious specificity for NOþ. One intriguing possibility is intermolecular Coulombic decay (ICD). ICD is a relatively recently discovered phenomenon in which ionization of an inner shell electron is followed by an electron on the same monomer dropping down to fill the hole.43,44 The energy released is then employed to eject an outer valence electron on a neighboring monomer unit, leading to Coulomb explosion and the possible production of relatively cold NOþ ions. Significantly, the double ionization potential of the cluster can be much lower than the monomer, particularly because two distinct sites are involved, and so this channel can open for clusters when it is closed for monomers. However, it is not clear whether or not the double ionization energy of (NO)2 and larger clusters will be lowered sufficiently relative to the monomer (38.48 eV)45 to allow such a mechanism to be energetically viable on charge transfer from Heþ, that is, at a threshold of about 24 eV. Furthermore, while ICD is highly unlikely for (N2)n and (CO)n because of their significantly higher double ionization thresholds, O2 has a lower double ionization threshold (36.13 eV)46 than NO, and thus, if ICD is operational for (NO)n, then it may also be viable for (O2)n. Consequently, the mechanism by which cold NOþ ions are produced from (NO)nþ clusters remains an open question.

’ CONCLUSIONS The production of helium-doped molecular cations from molecule-doped helium nanodroplets following electron impact ionization is compared for the dopants N2, CO, O2, and NO. A remarkable difference in behavior is observed, with monomer cations (Mþ) virtually stripped bare of helium atoms in the gas phase, whereas the corresponding diatomic clusters ions, Mnþ, show a much greater propensity for departing with helium atoms in tow. This contrasting behavior is explained by differential cooling following the initial ionization event. In particular, the cluster ions have the opportunity for more efficient cooling than the monomers for two reasons, both of which appear to be in operation for the diatomics selected here: (i) evaporative loss of monomers from larger Mnþ cluster ions; (ii) more effective channelling of excess energy into the helium matrix, most likely through the lower frequency intermolecular vibrational modes that are available in the clusters but not in the monomers. As well as explaining the experimental observations in the present work, such a scenario may also explain a previously noted propensity for helium atoms to bind preferentially to molecular cluster anions when low energy electrons are injected into doped helium nanodroplets. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected]. Tel.: þ44 (0)116 252 2138. Fax: þ44 (0)116 252 3789.

’ ACKNOWLEDGMENT The authors wish to thank the UK Engineering and Physical Sciences Research Council for funding and, particularly, the award of an Advanced Research Fellowship to S.Y. The authors 7015

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The Journal of Physical Chemistry A also appreciate the support from the National Service for Computational Chemistry Software (NSCCS), tendered by Imperial College, London.

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