ARTICLE pubs.acs.org/JPCA
Double Ionization and Coulomb Explosion of the Formic Acid Dimer by Intense Near-Infrared Femtosecond Laser Pulses Kennosuke Hoshina,* Hiroshi Hagihara, and Masashi Tsuge Faculty of Pharmaceutical Sciences, Niigata University of Pharmacy and Applied Life Sciences, 265-1, Higashijima, Akiha-ku, Niigata-city 956-8603, Japan ABSTRACT: Ionization and fragmentation of formic acid dimers (HCOOH)2 and (DCOOD)2 by irradiation of femtosecond laser pulses (100 fs, 800 nm, ∼1 1014 W/cm2) were investigated by time-of-flight (TOF) mass spectrometry. In the TOF spectra, we observed fragment ions (HCOOH)H+, (HCOOH)HCOO+, and H3O+, which were produced via the dissociative ionization of (HCOOH)2. In addition, we found that the TOF signals of COO+, HCOO+, and HCOOH+ have small but clear side peaks, indicating fragmentation with large kinetic energy release caused by Coulomb explosion. On the basis of the momentum matching among pairs of the side peaks, a Coulomb explosion pathway of the dimer dication, (HCOOH)22+ f HCOOH+ + HCOOH+, was identified with the total kinetic energy release of 3.6 eV. Quantum chemical calculations for energies of (HCOOH)22+ were also performed, and the kinetic energy release of the metastable dication was estimated to be 3.40 eV, showing good agreement with the observation. COO+ and HCOO+ signals with kinetic energies of 1.4 eV were tentatively assigned to be fragment ions through Coulomb explosion occurring after the elimination of a hydrogen atom or molecule from (HCOOH)22+. The present observation demonstrated that the formic acid dimer could be doubly ionized prior to hydrogen bond breaking by intense femtosecond laser fields.
’ INTRODUCTION Recent intensive research on the interaction between molecules and strong femtosecond laser pulses helped to develop a new concept for molecular responses to laser irradiation, which exceeds the perturbative regime.13 Ionization is one of the basic processes and exhibits a very different behavior than that observed in a weak photoelectric field. Owing to short-pulsed and intense laser fields, which have a duration shorter than several hundred femtoseconds and an intensity higher than 1013 W/cm2, ionization overcomes the competing dissociation processes, resulting in the multiple ionization of molecules before dissociation. In general, the produced multiply charged ions are unstable; therefore, they dissociate into fragments of atomic and molecular ions with high kinetic energy through the Coulomb explosion process.4 The molecular structure and spatial orientation of multiply charged ions just before Coulomb explosion have been reconstructed from the ejected fragment ions and recognized with respect to molecular alignment5 and structural deformation6 induced by the formation of the dressed state followed by enhanced ionization.4 These are characteristic features of isolated molecular systems in intense femtosecond laser ionization, although molecules can also be multiply ionized before fragmentation by the electron ionization7 and absorption of high-energy photons.8 For diatomic and small-sized polyatomic molecules, these phenomena have been intensively investigated in detail by using r 2011 American Chemical Society
time-of-flight (TOF) mass spectroscopy, two-dimensional imaging based on the TOF spectra,9 and, recently, ionion correlations.10,11 Unlike small molecules, the analysis of the fragmentation process becomes complex in molecular systems such as large organic molecules,12 biomolecules,13 aromatic molecules,14,15 and polycyclic molecules.16,17 Alternatively, the subject of interest in these large systems has been whether the dominant process is fragmentation or ionization and the competition between intramolecular energy flow and ionization.18 It has been demonstrated that, when the laser wavelength is resonant in the cation, strong coupling between electronic states via intense laser irradiation induces vibrational energy deposition within the molecule, which is then followed by fragmentation rather than further ionization.19,17,20,21 A molecular complex bounded by a weak intermolecular interaction is another important system of isolated molecules in intense femtosecond laser fields, because this system is considered to be an intermediate phase that links isolated molecules to the condensed system. From the biochemical perspective, complexes with hydrogen bonding are particularly important because hydrogen bonding plays a key role in the function of biomolecules. However, these systems have not yet Received: November 18, 2011 Revised: December 13, 2011 Published: December 14, 2011 826
dx.doi.org/10.1021/jp2111154 | J. Phys. Chem. A 2012, 116, 826–831
The Journal of Physical Chemistry A been intensively investigated. In the case of hydrogen-bonded complexes containing aromatic molecules, it was reported that the decomposition of aromatic moieties is suppressed, and an intercluster reaction occurs by the formation of hydrogenbonded complexes with H2O or NH3.22,23 Recently, we demonstrated that the ionization probability of ethanol was enhanced by dimer formation when using femtosecond laser irradiation, whereas the same was not observed when a nanosecond laser was used.24 For the multiple ionization probability, an enhancement was recognized by the formation of a trimer, which has a ring structure with three hydrogen bonds.24 In this study, we chose a formic acid dimer as the prototype system having plural hydrogen bonds for multiple molecular ionization by femtosecond laser pulses. This system is closely related to biomolecules such as the DNA base pair held together through multiple hydrogen bonding.25 On the basis of a recent theoretical research on the formic acid dimer, 11 stable conformers were expected and experimentally identified in a matrix isolation method.26,27 The binding energy of the well-known, most stable structure with C2h symmetry was estimated to be ∼0.6 eV.28 The major fragment ion in the dissociative ionization of the formic acid dimer by high-energy photoabsorption29 is reported to be (HCOOH)H+, which is produced in the proton-transfer reaction, whereas no information on multiple ionization is available. We measured the fragment ions produced by intense femtosecond laser irradiation with a field intensity of ∼1014 W/cm2 and a duration of 100 fs, and discussed fragmentation processes after double ionization by using quantum chemical calculations.
’ EXPERIMENTAL METHOD Output pulses (800 nm, ∼100 fs, 10 Hz,
