Ionization and fragmentation of formic acid dimers (HCOOH) _2 and (DCOOD) _2 by irradiation of femtosecond laser pulses (100 fs, 800 nm, ～1 × 10 ~(14) W/cm ~2) were investigated by time-of-flight (TOF) mass spectrometry. In the TOF spectra, we observed fragment ions (HCOOH)H ~+, (HCOOH)HCOO ~+, and H _3O ~+, 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) _2~(2+) → HCOOH ~+ + HCOOH ~+, was identified with the total kinetic energy release of 3.6 eV. Quantum chemical calculations for energies of (HCOOH) _2 ~(2+) 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) _2 ~(2+). The present observation demonstrated that the formic acid dimer could be doubly ionized prior to hydrogen bond breaking by intense femtosecond laser fields.
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