Formation of Molecular Halide Ions from Alkyl-Halide Clusters

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Formation of Molecular Halide Ions from Alkyl-Halide Clusters Irradiated by ps and fs Laser Pulses G. Karras, S. Danakas, and C. Kosmidis* Department of Physics, University of Ioannina, 45110 Ioannina, Greece ABSTRACT: We report on the interaction of alkyl-halide clusters with 35 ps and 20 fs laser pulses at λ = 266, 532, and 1064 nm and 400 and 800 nm, respectively. Particularly, we examine by means of time-of-flight mass spectrometry the intracluster photochemical processes, which give rise to the formation of molecular halogen ions. The efficiency of molecular halogen ion formation is found to depend strongly on the laser wavelength and pulse duration. The ionization/excitation schemes involve in both cases the multiphoton absorption by the clusters and the combined action of the laser and the intracluster electric field. Intracluster energy transfer processes seem to have a significant contribution to the molecular halogen ion formation in the ps domain, while in the fs region, this is probably facilitated by a rescattering process and/or by photon absorption. A physical mechanism for the interpretation of our experimental results is proposed.

I. INTRODUCTION The interaction of molecular clusters with laser radiation has attracted considerable interest in the past decade, especially with the advent of strong and ultrashort laser pulses.13 Due to their complexity, molecular clusters are very difficult to treat either theoretically or analytically. Nevertheless, the body of experimental work is substantial, even without matching that of atomic clusters. As far as the case of alkyl halide clusters is concerned, the vast majority of the reported data examines the interaction of methyl iodide clusters with nanosecond (ns), picosecond (ps), and femtosecond (fs) laser pulses at the typical wavelengths of the available laser sources. Several interesting phenomena raised by these studies such as Coulomb explosion at low and moderate laser intensities, production of highly charged ions and electrons with anisotropic angular distributions, and photochemical processes taking place within the cluster environment, to mention a few. The intracluster photochemical processes are the subject of the present work and more specifically the formation of molecular halogen ions from the interaction of alkyl halide clusters with laser irradiation. Sapers et al.4 have first reported the formation of molecular iodide ion from the interaction of methyl iodide clusters [(CH3I)n] with ns pulses. The ionic signal of I2 was attributed to the ionization of the neutral molecule. The latter is produced mostly, in that case, by the sequential dissociation of the two CI bonds of the monomers that form the neutral dimer. On the other hand, Parker et al.5 studying (CH3I)n under similar experimental conditions have argued that the photodissociation of the ionized dimer is the most probable precursor for the formation of I2þ. Recently, Kochubei et al.6 reported on the interaction of (HI) clusters with ns pulses and suggested that the r 2011 American Chemical Society

formation of I2þ is facilitated by the photodissociation (HI)2þ in a manner analogous to that reported from Parker et al. Syage and Steadman7 studied the interaction of methyl iodide clusters with electrons (electron impact at 20 and 60 eV) and laser pulses of picosecond pulse duration at 266 and 532 nm and they concluded that the origin of I2þ formation is the multiphoton ionization of the neutral molecular halogen which can be traced to the neutral clusters. Lockman et al. reported8 the production of I2þ from the irradiation of (CF3I) clusters with ns laser pulses. The authors attributed their findings to a photochemical reaction taking place within the cluster environment after its multiphoton ionization. Choi et al.9 attributed the formation of I2þ, from methyl iodide clusters irradiated at 266 and 355 nm with ns laser pulses, to intracluster photochemistry within the neutral manifold. Finally, Fan et al.10 investigating the interaction of several alkyl halide clusters with ns laser pulses by means of laser-induced fluorescence (LIF) verified the formation of neutral molecular iodide and not that of neutral molecular bromide. As it is clear from the above, the molecular halogen formation from the interaction of alkyl halide clusters with laser pulses is still a matter of controversy. In the present work, we report on the formation of molecular halogen ions from the interaction of some alkyl halide clusters with 35 ps and 20 fs laser pulses. Our study is focused on the molecular halogen ions ejection from methyl iodide clusters, which is a benchmark system, and from ethyl halide clusters (C2H5X)n (where X = I, Br, Cl). Received: February 17, 2011 Revised: March 16, 2011 Published: April 04, 2011 4186

dx.doi.org/10.1021/jp2015947 | J. Phys. Chem. A 2011, 115, 4186–4194

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Figure 1. Mass spectra of (CH3I)n produced by a 35 ps laser at 266 nm (3.3 1012 W/cm2). Dþ stands for dimer ions and Tþ stands for trimer ions.

II. EXPERIMENTAL SECTION Clusters of methyl iodide, ethyl iodide, ethyl bromide, and ethyl chloride molecules were produced via supersonic expansion in a vacuum chamber of a gas mixture through the 750 μm diameter orifice of a pulsed valve operating at 10 Hz. The gas mixture was prepared inside a bubbler containing each of the above molecules at a time at room temperature and with helium as carrier gas. Measurements were carried out also using a slash bath of ice and ethanol. In all cases, the backing pressure of the valve was held at two bars. The resulting molecular cluster beam reached the interaction region at 7 cm from the valve orifice. The interaction region was pumped by a diffusion pump (2400 L/s) equipped with a liquid nitrogen trap, and the rest of the chamber (field free region) was pumped by a turbo-molecular pump (100 L/s), resulting in a stagnation pressure below 2.6 105 Pa when the valve was closed. During the operation of the valve, the stagnation pressure was below 2.6 104 Pa. The interaction region was part of a time-of-flight mass spectrometer in a WileyMacLaren configuration using two acceleration stages for increased detection efficiency. The total length of the field free region after the second acceleration stage was 140 cm. The ionic signal was detected by a chevron type microchannel detector and recorded through a digital oscilloscope. Two separate laser systems were used to deliver the picosecond and femtosecond laser pulses to the interaction region. The former were provided by a Nd:YAG mode locked laser (Quantel YG 901C) that produced 35 ps pulses with 80 mJ energy per pulse at the fundamental wavelength of 1064 nm and 11 mJ after their passage through two frequency doubling crystals. Femtosecond laser pulses are provided by a Ti:Sapphire system (Coherent Legend Duo USX) operating at 1 kHz, which delivered near Fourier transform limited pulses centered at 790 nm, with maximum energy of 6 mJ and duration of 20 fs, as measured

with a single shot autocorrelator. The second harmonic of the fundamental 790 nm emerged after passing through a 150 μm thick BBO crystal. The laser beam was focused by a 50 cm focal length lens. The energy of each pulse was controlled using half wave plates and a thin film polarizer, while the polarization was selected using half and quarter wave plates. Time delay adjustments between the supersonic molecular beam and the laser pulses were achieved by using a digital delay generator (Stanford Research Systems DG535). The alkyl halide compounds were purchased from Aldrich (purity of >99.5%) and were used without further purification.

III. RESULTS The mass spectra of (CH3I)n induced by 35 ps laser pulses at 266, 532, and 1064 nm were recorded. Molecular iodide ions (I2þ) were observed only in the case of the 266 nm spectra (Figure 1). In the inset of Figure 1 it is depicted the ionic signal of some cluster ions along with that of I2þ. The dependence of the latter on the laser intensity is presented in Figure 2 (logarithmic axes), where it is seen that for laser intensities up to 3.3 1012 W/cm2 the slope is 2.5 and that for higher intensities the ionic signal saturates and starts to decrease. This dependence could be attributed to a three photon absorption process. Yet, the saturation and decrease of the ionic signal is indicative either of a dissociation of its precursors after their multiphoton ionization and/or of a competitive operation of other dissociative channels at higher laser intensities. In Figure 3, typical mass spectra of ethyl iodide and ethyl bromide clusters irradiated at 266 nm are presented. No Cl2þ ions were observed in the mass spectra of ethyl chloride clusters at all the wavelengths used, while I2þ and Br2þ ions were detected only in the mass spectra recorded at 266 nm 4187

dx.doi.org/10.1021/jp2015947 |J. Phys. Chem. A 2011, 115, 4186–4194


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Figure 2. Laser intensity dependence of I2þ ions, from (CH3I)n, for 35 ps laser pulses at 266 nm.

(they are depicted in the inserts of Figure 3). For the case of ethyl bromide and ethyl iodide, the main cluster ions are the Br2þ and the I2þ, respectively, while in the case of methyl iodide clusters, the ionic signal of the dimer and the demethylated dimer were quantitatively close to that of I2þ. The dependence of the molecular halogen ions on the laser intensity is presented in Figure 4 and shows a slope of 2.7 for Br2þ and 2.5 for I2þ, which is indicative for, at least, a three photon absorption processes. The appearance laser intensity thresholds (AI) of the molecular halogen ions for each molecular cluster are given in Table 1. Obviously, these values have not a universal validity since they are strictly related to the acceptance ion efficiency of the particular mass spectrometer used. Nevertheless, they make possible the comparison between the clusters under investigation. Thus, it can be seen that the appearance laser intensity thresholds increase with the size of the monomer molecular chain and the ionization energy of the neutral halogen. As for the interaction of alkyl halide clusters with 20 fs laser pulses it was studied at 800 and 400 nm and a sample of the produced mass spectra are shown in Figure.5. Once again no Cl2þ ions have been detected while Br2þ ions have been observed only in the mass spectra for 400 nm. In Figure 6 the dependence of the ionic signal of I2þ on the laser intensity is presented for both 800 and 400 nm for the case of methyl iodide clusters. The slope of the linear part of the graph

is ∼5.5, for the case of 800 nm, for laser intensities up to 1.2 x1013 W/cm2, while at higher intensities the signal decreases. Similar dependence on laser intensity for 800 nm is observed also in the I2þ ions ejected from (C2H5I)n clusters. This behavior indicates that, at least, a six photon absorption process is involved in the I2þ formation. In the case of 400 nm, the maximum available laser intensity was 4.5 1014 W/cm2 and the dependence of the I2þ signal on the laser intensity was measured to be cubic, 2.7, for the whole intensity range covered. In a recent publication we also reported on the angular distribution of the ejected atomic fragment ions from (CH3I)n following their interaction with 20 fs laser pulses.11 Three different types of angular distributions were detected and their close relation with the involved ionization mechanism was presented. For I2þ and Br2þ molecular ions of the present work, the angular distribution was found to be isotropic for both wavelengths used. Also, the appearance laser intensity thresholds, presented in Table 1, exhibit no dependence, within the experimental error, on the polarization (linear or circular) of the laser beam.

IV. DISCUSSION Before the analysis of the experimental data we had to make sure that the recorded molecular halogen ions do not come from 4188



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Figure 3. Mass spectra of (C2H5Br)n and (C2H5I)n induced by a 35 ps laser at 266 nm (6 1012 W/cm2).

traces of neutral I2 and Br2 molecules in the samples.12 Thus, we performed experiments by introducing the samples effusively in the interaction region via a small orifice.13 No molecular halogen ions have been detected, although experiments with significantly higher vapor pressure in the interaction region were also performed. Moreover, as seen in Table 1, the appearance laser intensity threshold of I2þ at 266 nm is higher for (C2H5I) clusters compared to (CH3I)n, which implies that I2 is not a common precursor. Furthermore, if I2 was the origin of the observed I2þ ions, the slope in the loglog plot of its dependence on laser intensity at 266 nm (4.66 eV per photon) should be quadratic (the IP of I2 is 9.3 eV14), which is not the case as we have seen. Therefore, the origin of the observed molecular halogen ions (X2þ) has to do with the photochemical processes taking place within the alkyl halide clusters. The next question to be answered is where these photochemical processes are taking place: in the neutral or in the ionic manifold? In the case of CH3I it is known that one-photon absorption at 266 nm leads to the excitation to dissociative electronic states (known as the A-band system). In these states, the dissociation time of the CI bond is determined to be about ∼170 fs15 and therefore, for the case of the 35 ps experiments, the formation of I2, which is successively ionized within the same laser pulse, cannot a priori be excluded. Nevertheless, it has been shown that the dissociation from these states has a negligible contribution to the recorded mass spectra of CH3I and (CH3I)n under the current ps irradiation conditions.16,17 This is also supported by the fact that the dependence of I2þ signal on laser intensity was found to be cubic at 266 nm (the total absorbed energy is at least 13.98 eV), implying that its formation is taking

place at energies higher than those needed for the ionization of dimer and trimer clusters (9.2 and 9.08 eV respectively18). Much more, in the case of the 20 fs experiments, there is not enough time for CI bond rupture and then formation of I2, followed by ionization within the same laser pulse. We can thus conclude that, in the present experiments, X2þ formation occurs in the cluster ionic manifold. Subsequently, it is reasonable to expect that the cluster ion geometry is affecting the efficiency of X2þ formation. Vidma et al4 have shown that in the case of (CH3I)nþ the formation of I2þ occurs mainly within the dimers. Ito et al.19 and Bogdanchikov et al.18 have identified two different stable arrangements for the monomers within the dimer: the Head to Head (H-H) geometry, which is more stable in the ionic manifold, and the Head to Tail (H-T) arrangement, being more stable in the neutral manifold. The ionization potential for the H-H dimer is 9.2 eV, while for the H-T, it is 9.74 eV.18 The presence also of both isomeric structures, in comparable amounts, in supersonic molecular beams has been verified experimentally.19 From a geometrical point of view, the smaller distance between the two halogens is expected to make the X2þ formation more probable for the H-H dimers. On the other hand, the production of X2þ from the H-T configuration could be rationalized via a delayed fragmentation process following the rotation of the monomer (the time needed is estimated at about 1 ps for the case of methyl iodide).18 In the fs experiments, the second process could take place only after the pulse end, but this is not supported from the present experiments. Having argued that the X2þ production is taking place in the ionic manifold of the H-H cluster configuration, this could happen via the 4189



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singly charged cluster is prerequisite for the X2þ formation. The question of the feasibility of these processes comes next, taking into consideration that X2þ formation has been also reported under low power ns laser irradiation as mentioned previously. Before any further discussion for the involved excitation processes though, it is of importance to refer to the conclusions of Nichols et al.20 who have studied the reactions between methyl halide cations and methyl halides. They found that in the [CH3X. 3 .XCH3]þ ionized dimer a two-center three-electron (2c-3e) bond is developed between the halogen atoms. For the case of methyl iodide, the I. 3 .I binding energy has been calculated to be 1.37 eV, while a value in the 0.991.12 eV range had been previously estimated for the same (2c-3c) bond.20,21 In any case, this binding energy is higher than that of a typical van der Waals bond and corresponds to more than 65% of a normal II bond. Obviously, the development of a (2c, 3e) bond, between the iodine or bromine atoms, is conceivable only in an HH cluster configuration. Nichols et al.20 have observed the formation of I2þ and Br2þ fragment ions from the collision-induced fragmentation of [CH3X. 3 .XCH3]•þ (X = I, Br), while no Cl2þ ions have been detected in the case of [CH3Cl-CH3Cl]•þ. For the last species, the existence of a 2c-3c bond between the chlorine atoms has not been safely confirmed; instead, they found evidence of a different atomic connectivity (for instance, [CH3ClHClCH2]•þ). The connection of the above findings with the results of the present work cannot be ignored. Cluster ionization, induced by the laser beam, could lead also to the formation of the same (2c3e) bonded ionic dimers, and the lack of Cl2þ ions from the reported mass spectra is indicative of a low formation probability of such a bond in alkyl-chloride clusters. For the case of the ionized methyl iodide dimers, the distance between the two halogens in the ground ionic state H-H configuration is estimated to be about 3.14 Å.18 To the best of our knowledge, there are not similar calculations in the literature for the rest of the alkyl-halide clusters under investigation. Moreover, the intermolecular distances in heavier clusters are expected to be much longer limiting, thus, the efficiency of a 2c-3e bond creation; therefore, we will focus our discussion on the (CH3I)2 case. Thus, the formation of X2þ ions via reaction 4 implies that the I fragment is released from the dissociation of the neutral monomer of a singly ionized dimer after one-photon resonant absorption. At the laser intensities used, this step is expected to be saturated and therefore does not affect the slope in the I2þ plot. A drawback of such a process is that there is not for all clusters studied a single-photon resonance at 266 nm. Obviously, this formation route of X2þ ions cannot be excluded, especially for methyl and ethyl iodide, but other alternatives, involving cluster’s double ionization, should also be explored. The creation of a I. 3 .I bond implies a contraction between the monomers in the ionic manifold. Because in the neutral H-H dimer the distance between the two iodine atoms is 3.8 Å,18 this readjustment takes some time, which depends on the kinetic energy

reaction X þ Xþ f X2 þ

ð1Þ

þ

where X and X are halogen fragments of the cluster’s constituent monomers. These fragments could be produced from the dissociative channels: Cn Hm Xþ f ðCn Hm Þþ þ X

ð2Þ

Cn Hm Xþ f ðCn Hm Þ þ Xþ

ð3Þ

C n Hm X f C n Hm þ X

ð4Þ

(where n = 1, 2 and m = 3, 5). The fragmentation via reactions 2 and ~ and B ~ excited states of the 3 are following the excitation to the A monomer ions, while the dissociation channel 4 could take place in the neutral manifold (from states resulting from the A monomer absorption band). Thus, it seems that ionization or excitation of a

Figure 4. Laser intensity dependence of I2þ and Br2þ ions from (C2H5I)n and (C2H5Br)n for 35 ps pulses at 266 nm.

Table 1. Appearance Intensity Thresholds (AI) for the Formation of X2þ Ions under Linearly and Circularly Polarized Laser Irradiation AI at 266 nm; linear polarization AI at 400 nm; linear polarization AI at 800 nm; linear polarization AI at 800 nm; circular polarization molecular halogen ion

(1012 W/cm2)

(1014 W/cm2)

(1014 W/cm2)

(1014 W/cm2)

I2þ from (CH3I)n

2 ( 0.1

1.7 ( 0.2

1.6 ( 0.2

1.6 ( 0.2

I2þ from (C2H5I)n Br2þ from (C2H5Br)n

4.5 ( 0.2 5.75 ( 0.2

2.6 ( 0.2 3.7 ( 0.3

2.8 ( 0.2

2.8 ( 0.2

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Figure 5. Mass spectra of (C2H5I)n and (C2H5Br)n induced by a 20 fs laser pulse at 400 nm (3.4 1014 W/cm2); Dþ stands for dimer ions.

(for free thermal motion this time is estimated to be more than 150 fs). Thus, for the 20 fs experiments, the dimer initial geometry is actually preserved during the pulse duration. On the contrary, under ns and ps pulse laser irradiation there is enough time for the readjustment, and finally, part of the laser pulse interacts with the dimers while the distance between the monomers is decreased. On the other hand, the presence of the single charged monomer produces a strong electric field around the second neutral monomer that lowers the electronic potential barrier between the molecules. Specifically, the electric field created by a positive charge at a distance of 3(4) Å is ∼1.6 (0.9) 1010 V/m. Such field strength can be produced by a laser beam of intensity ∼3.5 (1.1) 1013W/cm2, that is, one order of magnitude higher than the laser intensity thresholds observed in the present ps experiments and even more than those reported by using ns laser pulses. Thus, the suppression of the electronic potential, further enhanced by the laser field, increases the ionization probability, via electron tunnelling, leading finally to double ionization of the dimer. The higher laser intensity threshold for the 20 fs pulses is attributed to the expected weaker potential barrier suppression from the internal field, because the initial distance between the iodine atoms remains actually unchanged during such an ultrashort light pulse. Through the mechanism just described, the creation of a twocenter doubly ionized cluster is rationalized, but the creation of

Iþ and I fragments, a prerequisite for I2þ formation in accordance to reaction 1, remains unclear. It is well-known that under ns and ps excitation the power dependence of the signal on light intensity is affected, among others, by the presence of resonances with real states of the irradiated system. Often, it is found to be lower than the number of the absorbed photons when resonances are involved. For instance, in the present ps experiments, the slope for the linear part of Iþ signal plot versus light intensity was found to be 1.16, while it is known that this ion cannot be generated after two photon absorption via any possible process. In the ps experiments the slope of 2.5 for the I2þ indicates the absorption of at least three photons, leading to the excitation to ~ ionic state which leads to Iþ ejection (reaction 3). Further the B excitation offers the required energy for the dissociation of the ~ ionic state via the reaction channel 2. other monomer from the A It should be noted that from the photoelectron spectra is deduced that the absorption of an additional photon from singly ionized molecules is facilitated by resonances with ionic states for all the molecules studied.22 Thus, the X2þ ions could be the end products of a fragmentation process taking place in a doubly ionized dimer. Alternatively, in the case of a 4-photon direct excitation, the other monomer can be excited via intracluster energy transfer, that is, Intermolecular Coulombic Decay (ICD) and/or the so-called “trivial” processes.23 The role of ICD was 4191



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case, the ponderomotive energy for an electron liberated from the cluster can be estimated by the equation: Up ðeVÞ ¼ 9:33 1014 3 IðW=cm2 Þ 3 λ2 ðμmÞ

Figure 6. Laser intensity dependence of I2þ ions from (CH3I)n and for 20 fs pulses (a) 400 nm and (b) 800 nm.

reconsidered as a possible excitation mechanism in recent experiments with clusters irradiated with fs laser pulses at 800 nm,24,25 but its contribution in the formation of X2þ in the ps experiments cannot be excluded. For the case of 20 fs experiments, the laser intensity thresholds for X2þ ion formation are in the order of 1014W/cm2. At this intensity, level double ionization of the monomers has been previously reported26 and, as supported by our data, the same is valid for the case of clusters, too. This is confirmed by the fact that the determined laser intensity thresholds for the appearance of X2þ are lower than those of X2þ, indicating that the former are fragmentation products of, at least, a doubly ionized cluster. The latter are expected to be one-center doubly ionized clusters because the X2þ fragments possess complex peak profiles representing the kinetic energy release from Coulomb explosion process. In contrast, the creation of two-center doubly ionized clusters facilitates the formation of X2þ ions and is feasible due to the combined action of the strong electromagnetic field and the internal field formed after the single cluster ionization. As mentioned above, the effective potential barrier suppression, under 20 fs irradiation, is achieved at higher laser intensity, and this accounts for observed laser intensity thresholds of X2þ ions. The two-center doubly ionized dimers created through this process (electron tunneling via suppressed barriers) are, most probably, generated in their ground ionic state without the necessary internal energy that can lead to their fragmentation to Xþ and X. However, at this stage, the cluster ion can be excited by the rescattering mechanism and/or by one (400 nm) or two (800 nm) photon absorption of the monomer ions. In the former

At the laser intensity threshold for the observation of I2þ, Up is ∼9.6 eV (∼2.5 eV) at 800 (400) nm. Thus, ionized monomers can be excited by electron impact at 800 and 400 nm even in a single step since the maximum energy of the rescattered electron ~ and B ~ states are lying ∼2.5 and is 3.17Up (in methyl iodide the A ∼4 eV, respectively, above the ionic ground state). The fact that the laser intensity thresholds at 800 nm for linear and circular polarization are identical (Table 1) is not in conflict with the rescattering mechanism because the process is taking place in an extended system.27 Clearly, such a X2þ formation process is conceivable only within a narrow range of laser intensities. The lower limit of this range is defined by the intensity needed for the two-center double cluster ionization (combined action of the laser and the internal field), while the upper one from the necessity that the Up energy for the rescattered electron is not enough to lead to further ionization (multielectron ionization implies strong repulsive forces and subsequent coulomb explosion which prevent reaction 1). As far as the observed slope in the plot of I2þ versus 20 fs laser intensity is concerned, we should note that this reflects the order of the multiphoton absorption leading in the single ionization of clusters. Under 20 fs irradiation, the influence of possible resonances with real states is expected to be negligible as it is experimentally confirmed.28,29 Even in the case that the created two-center doubly ionized dimers are excited by one (400 nm) or two (800 nm) photon absorption, the slope will be dictated by the higher order multiphoton ionization process. A representation of the proposed mechanism leading to molecular halogen ion is depicted in Scheme 1a. Unfortunately, for the ethyl iodide and ethyl bromide clusters, the literature information is poor compared to that for methyl iodide. Nevertheless, it is known that the appearance energy (AE) of Iþ from C2H5I is 14.8 ( 0.2 eV, while for Brþ from C2H5Br is 18.6 ( 0.3 eV. The higher AE value for Brþ is in accordance with the higher laser intensity thresholds for Br2þ formation observed for the ethyl bromide clusters (Table 1). Additionally, the absence of Br2þ ions from the mass spectra recorded at 800 nm can be attributed to the high AE value for Brþ ejection because its formation by electron impact excitation, in one step, can take place at higher laser intensities, which would induce multiple ionization and the Coulomb explosion of the cluster, preventing the Br2þ ion formation.30 On the contrary, the appearance of Br2þ ions in the mass spectra recorded at 400 nm underlies the possibility of the ethyl bromide monomer excitation through the already proposed processes of photon absorption or the rescattered electron impact.

V. CONCLUSIONS The formation of I2þ and Br2þ ions resulting from the interaction of (CH3I)n and (C2H5X)n (X = I, Br) with ps and fs laser pulses, is reported. Under the same irradiation conditions, no Cl2þ ions have been observed from (C2H5Cl)n. The creation of molecular halide ions is favored in clusters with a H-H configuration. The lack of Cl2þ from the mass spectra is attributed to the fact that instead of a (2c-3e) bond between the chlorine atoms, the bonding of the ethyl chloride monomers within the cluster takes place via a different atomic connectivity. 4192



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Scheme 1. Proposed Mechanism of the Molecular Halogen Ions Formation for (CH3I)2 for (a) Femtosecond and (b) Picosecond Laser Pulsesa

a

MEDI and CE stand for multielectron dissociative ionization and coulomb explosion, respectively.

Our experimental results indicate that the formation of X2þ takes place in the ionic manifold, and the proposed mechanism is concomitant to the laser’s pulse duration. Under ps irradiation, there is enough time for the clusters to form a (2c-3e) bond in the singly ionized manifold, which results in an increase of the internal field, and via intracluster energy transfer processes and/or one-photon further absorption, doubly ionized/excited dimers are formed that upon fragmentation lead to X2þ formation. On the other hand, under 20 fs pulses irradiation the molecular halogen ions formation is facilitated by the fragmentation of a two-center doubly ionized cluster, formed by the combined action of the laser and the cluster internal electric field. The excitation scheme of the ionized species involves the rescattering mechanism and/or a multiphoton absorption process. The proposed mechanisms are in agreement with the experimental findings of the present work. Nevertheless, due to the limited resolution of the recorded mass spectra in the ions of interest, it was not possible to gain any further information from the released kinetic energies of these fragments. This piece of information could be provided by photoelectron spectroscopy and is expected to solidify the proposed mechanism.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ30 26510 08537. Fax: þ30 26510 08695. E-mail: [email protected].

’ ACKNOWLEDGMENT We would like to express our thanks to the Central Laser Facility of the University of Ioannina for their facilities and their assistance. ’ REFERENCES (1) Fennel, Th.; Meiwes-Broer, K.-H.; Tiggesb€aumker, J.; Reinhard, P. G.; Dinh, P. M.; Suraud, E. Rev. Mod. Phys. 2010, 82, 1793–1842. (2) Hertel, I. V.; Radloff, W. Rep. Prog. Phys. 2006, 69, 1897–2003. (3) Dermota, T. E.; Zhong, Q.; Castleman, A. W., Jr. Chem. Rev. 2004, 104, 1861–1886. (4) Sapers, S .P.; Vaida, V.; Naaman, R. J. Chem. Phys. 1988, 88, 3638–3645. (5) Vidma, K. V.; Baklanov, A. V.; Khvorostov, E. B.; Ishchenko, V. N.; Kochubei, S. A.; Eppink, A. T. J. B.; Chestakov, D. A.; Parker, D. H. J. Chem. Phys. 2005, 122, 203401. (6) Vidma, K, V.; Parker, D. H.; Bogdanchikov, G. A.; Baklanov, A. V.; Kochubei, S. A. J. Phys. Chem. A 2010, 114 (9), 3067–3073. (7) Syage, J. A.; Steadman, J. Chem. Phys. Lett. 1990, 166, 159–166. (8) Lokhman, V. N.; Ogurok, D. D.; Ryabov, E. A. Chem. Phys. 2007, 333, 85–95. (9) Choi, Y. K.; Koo, Y. M.; Jung, K. W. J. Photochem. Photobiol., A 1999, 127, 1–5. (10) Fan, Y. B.; Randall, K. L.; Donaldson, D. J. J. Chem. Phys. 1993, 98, 4700–4706. (11) Karras, G.; Kosmidis, C. Chem. Phys. Lett. 2010, 499, 31–35. (12) Calvert, J. G. ; Pitts, J. N. Photochemistry; Wiley: New York, 1966. 4193



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Formation of Molecular Halide Ions from Alkyl-Halide Clusters

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