wavelength-dependent coulomb explosion in carbon disulphide (cs2) clusters: generation of energetic...

RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2007; 21: 2663–2670

) DOI: 10.1002/rcm.3127
Published online in Wiley InterScience (www.interscience.wiley.com
Wavelength-dependent Coulomb explosion in carbon

disulphide (CS2) clusters: generation of energetic

multiply charged carbon and sulphur ions

Pramod Sharma and Rajesh K. Vatsa*Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

Received 2 April 2007; Revised 1 June 2007; Accepted 1 June 2007

*CorrespoAtomic RE-mail: rk

A gigawatt laser-induced Coulomb explosion has been observed in carbon disulphide (CS2) clusters

generating energetic, multiply charged [C]mR (m ¼ 1–4) and [S]nþ (n ¼ 1–6) atomic ions of carbon and

sulphur. The Coulomb explosion shows wavelength dependence. Comparison of these results with

our earlier work shows that the polarizability and dipole moment might help in energy absorption

from the laser field but they are not mandatory conditions for this low-intensity Coulomb explosion.

The results show that in a field of 109 W/cm2, absorption of 266 and 355 nm laser radiation by CS2

clusters leads to multiphoton dissociation/ionization whereas at 532 nm the whole cluster explodes

generating multiply charged atomic ions. Copyright # 2007 John Wiley & Sons, Ltd.

In our previous work,1 Coulomb explosion was observed in

methyl iodide clusters under laser intensity condition of

�109W/cm2 as opposed to a previously reported intensity

of 1011W/cm2 or higher.2,3 Based on wavelength-dependent

studies, it was concluded that intermediate excited states

played an important role at these power levels.1 However,

synergistic contribution from the large size of the iodine atom

and hence ease of polarizability as well as from the dipole

moment, which can efficiently couple laser energy with the

cluster and lead to Coulomb explosion in (CH3I)n at gigawatt

laser intensities, could not be ruled out. In order to

investigate the role of excited electronic states, dipole

moment and polarizablity, further experiments were carried

out with compounds devoid of iodine and having zero

dipole moment in the ground state. Carbon disulphide (CS2),

which is a linear molecule in the ground electronic state with

net zero dipole moment and sufficient vapor pressure, was

chosen. CS2 also has an extensive absorption spectrum from

the vacuum-ultraviolet (VUV) to the near visible region and

the excited states could be accessed by multiphoton

excitation using the harmonics of a Nd:YAG laser.

A number of studies employing electron and photon

excitation techniques have been carried out on CS2 and its

clusters (CS2)n to understand the fragmentation dynamics,

spectroscopy and photochemistry. Previous spectroscopic

studies on the CS2 molecule have revealed the complexity of

its excited electronic states.4 Seaver et al.5 have proposed an

ion-molecule reaction mechanism to explain the formation of

the [CS2]þ ion from the predissociative state of CS2 ( ~A

1B2) as a

result of their multiphoton fragmentation and ionization

study of CS2 in the UV region. Resonance-enhanced multi-

photon ionization (REMPI) studies carried out on CS2permitted identification of different ns, np, nd, and nf

ndence to: R. K. Vatsa, Chemistry Division, Bhabhaesearch Centre, Mumbai 400 085, [email protected]

Rydberg states. In the two-photon resonant spectra,6,7

transitions to ns and nd gerade Rydberg states are allowed

and indeed have been observed, while in the three-photon

resonant spectra the np and nf ungerade Rydberg states have

been also identified.8–10 Mathur and coworkers11,12 have

investigated wavelength-dependent single and multiple

ionization of the CS2 monomer by intense picosecond laser

fields at five discrete wavelengths ranging from the IR to the

UV region. In these studies, in terms of the Keldysh

adiabaticity parameter (g), the multiphoton and the tunnel-

ling regimes were both covered (g � 0.3, 3, 8) since, for

g >> 1, the ionization of a molecule is attributed to a

multiphoton ionization process while, for g << 1, it is

attributed to a field ionization mechanism. It was concluded

that the dynamics of the dissociative ionization process are

dependent on the regime in which the laser–molecule

interaction occurs. Using 50 fs pulses in the near-IR

wavelength region (790 nm), Smith et al.13 observed triply

charged ions of CS2 at an intensity of 1015W/cm2. However,

upon increasing the laser intensity to �1016W/cm2, the

appearance of multiply charged atomic species (up to C4þ

and S6þ) was observedwhichwas taken as the onset intensity

for Coulomb explosions at this wavelength.

Michalak and Pelc14,15 studied the dynamics of CS2 cluster

formation by adiabatic gas expansion of CS2 vapor seeded

in argon (Ar) as a function of pressure/temperature and

measured the appearance potential for different fragments of

cluster ions using electron ionization. It was found that the

appearance potentials for clusters n� 9 decrease with

increasing value of n. Patsilinakou et al.16 studied the

fragmentation dynamics of multiphoton excited CS2 and its

clusters under free jet expansion in the wavelength region of

329–345 nm by employing time-of-flight (TOF) mass

Copyright # 2007 John Wiley & Sons, Ltd.

2664 P. Sharma and R. K. Vatsa

analysis. From these studies they concluded that, although

the photochemistry of the monomer does not exhibit

pronounced wavelength dependence in this region, the

fragmentation dynamics of CS2 clusters produced in the jet

depend strongly on wavelength.

In this communication, we present our results on

wavelength-dependent Coulomb explosion in CS2 clusters

under gigawatt intensity. We further show that the

absorption of higher energy photons leads to fragmentation

rather than Coulomb explosion, as opposed to the conven-

tional wisdom.

EXPERIMENTAL

Details of the experimental setup have been described in our

earlier publications1,17–19 and only a brief account is given

here. Neutral clusters of CS2 were generated via supersonic

expansion of room-temperature CS2 vapors seeded in

helium/argon with different backup pressures varying from

1 to 5 bar. The experimental results were found to be

independent of carrier gas, and so all the experimental

results presented here correspond to helium carrier gas. A

pulsed valve (0.6mm nozzle diameter and 300 ms pulse

duration) was used for this purpose. The molecular beam

produced in this waywas skimmed at a distance of 5 cm from

the pulsed nozzle. Ionization and Coulomb explosion were

carried out using second, third and fourth harmonics of a

pulsed Nd:YAG nanosecond laser (Quantel, Les Ulis, France:

model YG 980 E, 8 ns pulse width). The distance between

skimmer and ionization region was 17 cm. The ions so

Figure 1. TOF mass spectra of CS2 upon

355nm, under cluster conditions. Small peak

mass side of each of the major sulphur-co

of the 34S isotope (4.2% natural abundan

500 laser shots.


formed were accelerated and guided into a 100 cm field-free

region using a double focusing Wiley-McLaren assembly

and detected using a channel electron multiplier (CEM)

detector (Dr. Sjuts Optotechnik GmbH, Gottingen,

Germany). Typical voltages applied to repeller and extractor

grids were 2650V and 1000V, respectively. The ion signal

from the CEM detector is transferred to a digital oscilloscope

for averaging and further processed on a computer. Themass

resolution of the instrument is �300.

The ion signal was optimized by varying the delay

between pulse-valve opening and laser firing so that the laser

pulse interacts with the clustered portion of the gas pulse.

Experiments involving change in laser energy were also

carried out. Typically 500 laser shots were averaged for

every TOF spectrum. For weaker signals, 1000–1500 shots

were averaged.

RESULTS AND DISCUSSION

Multiphoton ionization and fragmentationof (CS2)n at 355 nmFigure 1 shows TOFmass spectra obtainedwhenCS2 clusters

were irradiated with 355 nm laser pulses having intensity

�1.6� 109W/cm2. As can be seen from the figure, ions atm/z

12, 32, 44, 64 and 76, corresponding to [C]þ, [S]þ, [CS]þ, [S2]þ

and [CS2]þ, respectively, are observed in the mass spectra.

The small peaks evident on the higher mass side of each of

the major sulphur-containing ions in Fig. 1 are due to the

presence of the 34S isotope (4.2% natural abundance).

Although cluster fragment ions could not be observed in

irradiation with 3.5 mJ laser energy at

s (marked with �) evident on the higher

ntaining ions are due to the presence

ce). The spectrum was averaged for


DOI: 10.1002/rcm

Figure 2. TOF mass spectra of CS2 upon irradiation at 355 nm, under monomer

conditions, i.e. by ionizing at the leading edge of the gas pulse.

Wavelength-dependent Coulomb explosion in CS2 clusters 2665

the mass spectra, the presence of [S2]þ is a signature for the

formation of CS2 clusters as no [S2]þ ions were observed

under monomer conditions (Fig. 2). These Sþ2 ions arise from

ion-molecule reactions, which occur within the cluster.5

From the mass spectra, it is clear that clusters of CS2 undergo

multiphoton fragmentation/ionization at 355 nm. No signa-

ture of the occurrence of Coulomb explosion, i.e. observation

of multiply charged atomic ions with large kinetic energy,

could be detected at this wavelength even up to laser

intensities of �1010W/cm2.

Irradiation of (CS2)n clusters at 532 nm:observation of Coulomb explosionIn our previous study, methyl iodide clusters showed

pronounced wavelength dependence for the occurrence of

Coulomb explosion. Thus, to investigate whether CS2clusters show a similar behavior, further studies were

carried out at 532 nm using the second harmonic of the

YAG laser under identical intensity conditions (�109W/

cm2).

When (CS2)n clusters were subjected to gigawatt laser

intensity pulses at 532 nm, formation of multiply charged

atomic ions of sulphur and carbon was observed. Figure 3

shows a typical TOFmass spectrum of CS2 clusters irradiated

with gigawatt intensity laser pulses of 532 nm. In addition to

the [CS2]þ molecular ion (m/z 76) and fragment ions like

[CS]þ and [S2]þ (Fig. 3(b)), broad asymmetric peaks atm/z 32,

16, 10.6, 8, 6.4, 5.3, 12, 6, 4 and 3were also observed (Fig. 3(a)).

These broad asymmetric peaks were assigned to m/z values

corresponding to [S]nþ (n¼ 1–6) and [C]mþ (m¼ 1–4). Under

our experimental conditions we were unable to detect

fragment ions of clusters, but the TOF mass spectra recorded

as a function of delay between the pulsed-valve opening and

laser pulse firing (Fig. 4) clearly suggest that these multiply


charged ions of sulphur and carbon are observed only when

the clustered portion of the molecular beam interacts with

the laser pulse.

Thus, our studies on clusters of CS2 suggest that they show

behavior similar to that of CH3I clusters, which also exhibit

Coulomb explosion phenomena at 532 nm,while undergoing

multiphoton dissociation/ionization at 355 nm. In order to

understand this wavelength dependency of the Coulomb

explosion phenomena, laser power dependency studies

on CS2 cluster at 355 and 532nm were carried out, and

these throw light on the role of the intermediate excited

states. The results are presented below.

Laser power dependence studies and kineticenergy of multiply charged atomic ionsproduced during Coulomb disintegrationof CS2 clustersBased on the Keldysh parameter (g >> 1),1,20 the present

studies fall in the regime of multiphoton ionization. The

minimum number of photons required for CS2 ionization at

355 and 532nm is at least 3hn and 5hn, respectively.

Additionally, at 532 nm multiply charged ions with kinetic

energy up to 600 eV (Table 1) and with an ionization energy

(IE) as high as 64.5 eV (C4þ) and 88 eV (S6þ) (Table 2) were

observed. This suggests that large numbers of photons from

the 532 nm laser pulsemust be efficiently absorbed by the CS2cluster. The laser power dependency studies carried out at

355 nm on different fragment ions of the CS2 cluster,

however, showed a two-photon dependency, indicating that

ionization is mediated through a resonant excitation state at

7 eV (Fig. 5). Similarly, the laser power dependence studies

carried out at 532 nm for different fragment ions of the CS2cluster gave a slope of 4 (Fig. 6). This suggests that, at 532 nm,

the ionization is mediated via resonant excited state at


DOI: 10.1002/rcm

Figure 3. TOF mass spectra of CS2 clusters at 532 nm with laser intensity

�4.6� 109W/cm2. For clarity the figure has been divided into twomass regions: (a)

m/z 0–40 and (b) m/z 40–85. Inset of (a) shows the enlarged portion in the

mass range m/z 2–6. f and b denote the forward and backward component of

given mass peak.


9.32 eV and that this forms the bottleneck in the overall

absorption process. Morgan et al.10 have shown that there are

Rydberg states in the vicinity of the 9.32 eV energy range

corresponding to the ½1=2�6ssg state which could only be

accessed by an even number of photons due to the selection

rules.

Although these studies point towards the important role

played by the intermediate excited states in inducing

Coulomb explosion, the exact mechanism of laser–matter

interaction operating in molecular clusters at 109W/cm2 is at

present unclear. It is worth mentioning here that the

mechanism and dynamical aspects of laser-induced ioniz-

ation of noble gas atoms in a strong laser field are considered

to be well understood.22 However, much less theoretical

analysis of the same process has been carried out on

complex systems such as diatomic, polyatomic molecules

and clusters. One of the main difficulties in such a treatment

is the still limited knowledge about ionization and fragmen-

tation pathways of large molecules/clusters in an intense

field. Our study which shows generation of highly charged


states (Coulomb explosion) at gigawatt laser intensities

together with wavelength dependency has added another

piece of useful information. The overall mechanism is not

known yet, however, and more experimental and theoretical

work is clearly needed before an unambiguous picture

can emerge.

Multiphoton ionization studies on CS2 clustersat 266 nmAs discussed above, laser power dependency studies carried

out on CS2 clusters at 532 nm suggested that a four-photon

absorption process to the intermediate excited level plays a

crucial role in inducing Coulomb explosion process in CS2clusters. As four photons of 532 nm are equivalent to two

photons of 266 nm in terms of energy and the selection rules

for the two-photon and four-photon processes are the same,

it is expected that 266 nm laser radiation will also induce

Coulomb explosion in clusters of CS2. Figure 7 shows mass

spectra of CS2 clusters irradiated with pulse energy in the

range of 1–10 mJ. In this energy range only fragment ions


DOI: 10.1002/rcm

Figure 4. TOF mass spectra obtained for CS2, as a function of delay between

pulsed-valve and laser firing at 532 nm. As can be seen from the figure the signal

intensity of multiply charged carbon and sulphur ions (C2þ and S2þ) is amaximum at

the time delay (500ms) where the no. density of clusters is a maximum.

Table 1. Kinetic energies of different ions produced upon Coulomb explosion of CS2 clusters calculated using the

formula Ekin¼ 9.65� 10–7Dt2n2F2/8m, where Dt is time difference (ns) between the split mass peaks, F (V/cm) is electric field

for ion extraction, n is the charge, and m (m/z units) is the mass of the fragment2,3

Sr. no. Laser energy

Kinetic energy of multiply charged ions (eV)

Sþ S2þ S3þ S4þ Cþ C2þ C3þ

1. 12mJ 13 74 484 602 15 170 638


along with a very weak signal for the parent molecular ion

(CSþ2 , m/z 76) at higher pulse energies were detected and no

Coulomb explosion could be observed. Thus, our studies

revealed that at 266 nm clusters of CS2 exhibit multiphoton

dissociation/ionization behavior similar to that of 355 nm

and unlike at 532 nmwhere it undergoes Coulomb explosion.

Laser power dependency studies (Fig. 8) still, however,

suggested a (2þ1) absorption process which leads to

formation of different ions at 266 nm. A possible reason

Table 2. Ionization energy of different charged states of

carbon and sulphur, observed at 532 nm21

Sr. no Charged states

IE of carbonto different

charged states

IE of sulphurto different

charged states

1 1þ 11.26 10.362 2þ 24.38 23.333 3þ 47.88 34.794 4þ 64.49 47.225 5þ 72.596 6þ 88.05

IE: ionization energy.


why CS2 clusters do not undergo Coulomb explosion at

266 nm is given below.

From Fig. 7 it can be seen that, at low laser energy, the

signal corresponding to the molecular ion is negligible in the

TOF mass spectra. With increasing pulse energy, a small

signal corresponding to the molecular ion was observed. The

fragmentation pattern observed in the mass spectra could be

interpreted as arising due to dominance of the ladder

switching process, i.e. extensive dissociation followed by the

ionization of the dissociated fragments. This leads to an

insignificant signal corresponding to the molecular ion at

lower laser energy. As the laser energy is increased, however,

the ladder climbing mechanism starts competing with the

ladder switching mechanism leading to formation of [CS2]þ,

which is observed in the mass spectra. In polyatomic

molecules, this competition between photodissociation and

photoionization is intensity dependent and is interesting in

the sense that it governs the final outcome. In the present

study, absorption of a single photon may be sufficient to

dissociate CS2, but intensity-dependent above-threshold

absorption of additional photons can lead to ionization

and dissociation on a higher potential energy surface.


DOI: 10.1002/rcm

Figure 5. Laser power dependency (ln-ln plot) for various ions: (a) CSþ2 , (b) S

þ2 , (c) C

þ, and (d) Sþ ions,

formed upon interaction of CS2 clusters with 355 nm laser pulse.

Figure 6. Laser power dependency (ln-ln plot) for various ions: (a) Sþ, (b) Cþ, (c) S4þ, and (d) C2þ ions, produced

upon interaction of CS2 clusters with 532 nm laser pulse.

Copyright # 2007 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2007; 21: 2663–2670

DOI: 10.1002/rcm


Figure 7. TOF mass spectra of CS2 clusters upon irradiation at 266 nm as a function of laser energy.

Small peaks marked with � and evident on the higher mass side of each of the major sulphur-containing

ions are due to the presence of the 34S isotope (4.2% natural abundance).

Figure 8. Laser power dependency (ln-ln plot) for various ions: (a) Cþ, (b) Sþ, and (c) CSþ ions, produced upon

interaction of CS2 clusters with 266 nm laser pulse.

Copyright # 2007 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2007; 21: 2663–2670

DOI: 10.1002/rcm



In the light of this observation, we conclude that at

266 nm CS2 molecules undergo dissociation upon absorption

of one photon via dissociation channel (1) with threshold

energy of 4.46 eV:23

CS2 þ hnð266 nmÞ�! CSðX1SþÞ þ Sð3PÞ (1)

These dissociation products absorb additional photons

from the laser pulse to undergo ionization via a (2þ1)

absorption process, which is reflected in the laser power

dependency studies carried out at 266 nm. This results in

inefficient population of the Rydberg state, which is

responsible for occurrence of Coulomb explosion at

532 nm. Channel (1) is also accessible upon absorption of

two photons of 532 nm laser but, as the selection rules for

one-photon (in this case 266 nm) electronic transition are

different from the selection rules for two-photon electronic

transition (in this case two photons of 532 nm) in the

centro-symmetric CS2 molecule,10 at 532 nm dissociation via

channel (1) seems to be insignificant as manifested by the

difference in the TOF spectra of Figs. 3 and 7.

CONCLUSIONS

Coulomb explosion has been observed for CS2 clusters at

532 nm, using nanosecond laser pulses of intensity �109W/

cm2, resulting in the formation of multiply charged atomic

ions up to S6þ and C4þ. A four-photon excitation to Rydberg

states in the energy region of 9.32 eV seems to be the

bottleneck for the occurrence of Coulomb explosion at

532 nm. The excited electronic states in the energy region of

9.32 eV can also be accessed by two photons of 266 nm, but

dominance of the dissociation process from the one-photon

excitation level at 266 nm seems to result in inefficient

population of Rydberg energy levels at 9.32 eV and hence

non-observance of Coulomb explosion at 266 nm.


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DOI: 10.1002/rcm

wavelength-dependent coulomb explosion in carbon disulphide (cs2) clusters: generation of energetic...

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