Ryuji Itakura

Dissociative ionization of ethanol in chirped intense laser fields

The dissociative ionization of ethanol C2H5OH in an intense laser field is investigated with a chirped laser pulse. From the sensitive dependence of the relative yields of the fragment ions on the absolute values of the linear chirp rate, it is shown that the light-dressed potential-energy surface (LDPES) at the singly charged stage governs the nuclear dynamics, and that the nuclear wave packet flow into the breaking of either of the C–C and C–O chemical bonds could be characterized by the holding time thold during which the LDPESs are maintained. It is also understood in term of the holding time that the enhanced ionization into the doubly charged stage followed by the Coulomb explosion at C–C or C–O proceeds when the nuclear wave packet at the singly charged stage reaches the critical distance for the further ionization.

Ionization and fragmentation dynamics of benzene in intense laser fields by tandem mass spectroscopy

Using a tandem time-of-flight mass spectrometer, benzene cations produced by the resonantly enhanced multiphoton ionization are mass separated and are exposed to intense laser fields (∼2×1016u200aW/cm2) at λ∼790 and 395 nm with the pulse duration of ∼50 fs. Comparing the yields of the product ions with those obtained from neutral benzene molecules, the ionization and dissociation dynamics of benzene in intense laser fields is investigated. At λ∼790 nm, the formation of parent benzene ions is a dominant process irrespective of the initial charge states, i.e., major products obtained when starting from neutral benzene are benzene cations and dications and those obtained when starting from benzene cation are benzene dications. On the other hand, at λ∼395 nm, the fragmentation processes to produce C4Hi+(i=2–4) and C3Hj+(j=1–3) dominate over further ionization to the benzene dication for both cases starting from neutral benzene and benzene cation, indicating the population trapping occurs by the efficient confinem...

Two-body Coulomb explosion and hydrogen migration in methanol induced by intense 7 and 21fs laser pulses

Two-body Coulomb explosion with the C-O bond breaking of methanol induced by intense laser pulses with the duration of Delta t=7 and 21 fs is investigated by the coincidence momentum imaging method. When Delta t=7 fs, the angular distribution of recoil vectors of the fragment ions for the direct C-O bond breaking pathway, CH(3)OH(2+)-->CH(3) (+)+OH(+), exhibits a peak deflected from the laser polarization direction by 30 degrees -45 degrees , and the corresponding angular distribution for the migration pathway, CH(2)OH(2) (+)-->CH(2) (+)+H(2)O(+), in which one hydrogen migrates from the carbon site to the oxygen site prior to the C-O bond breaking, exhibits almost the same profile. When the laser pulse duration is stretched to Delta t=21 fs, the angular distributions for the direct and migration pathways exhibit a broad peak along the laser polarization direction probably due to the dynamical alignment and/or the change in the double ionization mechanism; that is, from the nonsequential double ionization to the sequential double ionization. However, the extent of the anisotropy in the migration pathway is smaller than that in the direct pathway, exhibiting a substantial effect of hydrogen atom migration in the dissociative ionization of methanol interacting with the linearly polarized intense laser field.

Ejection dynamics of hydrogen molecular ions from methanol in intense laser fields

The ejection of hydrogen molecular ions from two-body Coulomb explosion processes of methanol (CH3OH, CD3OH and CH3OD) in an intense laser field (800 nm, 60 fs, 0.2 PW cm−2) is investigated by a coincidence momentum imaging method. From the coincidence momentum maps, the ejection processes of hydrogen molecular ions, CH3OH2+→ Hm+ + CH(3−m)OH+(m = 2, 3), CD3OH2+→ Dm+ + CH(3−m)OH+(m = 2, 3) and CH3OD2+→ Hm+ + CH(3−m)OD+(m = 2, 3), are identified. Based on the results obtained with isotopically substituted methanol, the isotope effect on the ejection process of hydrogen molecular ions is discussed. Furthermore, the ejection of H/D exchanged hydrogen molecular ions (HD+, HD2+ and H2D+) is identified, and the timescales for the H/D exchanging processes are estimated from the extent of anisotropy in the ejection directions.

Open-loop and closed-loop control of dissociative ionization of ethanol in intense laser fields

The relative yield of the C-O bond breaking with respect to the C-C bond breaking in ethanol cation C2H5OH+ is maximized in intense laser fields (10(13)-10(15) Wcm2) by open-loop and closed-loop optimization procedures. In the open-loop optimization, a train of intense laser pulses are synthesized so that the temporal separation between the first and last pulses becomes 800 fs, and the number and width of the pulses within a train are systematically varied. When the duration of 800 fs is filled with laser fields by increasing the number of pulses or by stretching all pulses in a triple pulse train, the relative yield of the C-O bond breaking becomes significantly large. In the closed-loop optimization using a self-learning algorithm, the four dispersion coefficients or the phases of 128 frequency components of an intense laser pulse are adopted as optimized parameters. From these optimization experiments it is revealed that the yield ratio of the C-O bond breaking is maximized as far as the total duration of the intense laser field reaches as long as approximately 1 ps and that the intermittent disappearance of the laser field within a pulse does not affect the relative yields of the bond breaking pathways.

Suppression of decomposition of aniline cation in intense laser fields by cluster formation with ammonia molecules

Mass-selected aniline cations and [aniline-(NH3)n]+ (n=1 and 2) cluster ions are exposed to the femtosecond laser fields (λ∼395u2009nm,I∼4×1015u2009W/cm2) and the nanosecond laser fields (λ=532u2009nm,I∼2.7×1010u2009W/cm2) by using a tandem type time-of-flight mass spectrometer. In the case of the bare aniline cation, the decomposition forming the five-membered ring compound, cyclopentadienyl cation (C5H6+), dominantly proceeds in both the femtosecond and nanosecond laser fields. When one or two ammonia molecules are attached to the aniline cation, the decomposition is significantly suppressed. This suppression was interpreted in terms of an intermolecular energy flow through the hydrogen bonding.

Dissociative ionization of ethanol by 400 nm femtosecond laser pulses

The dissociative ionization of ethanol in short-pulsed laser fields at approximately 400 nm is investigated. The yield ratio of the C-O bond breaking with respect to the C-C bond breaking increases sharply as the temporal width increases from 60 to 400 fs, and the yield ratio is two to three times as large as that at 800 nm in the entire pulse-width range of 60-580 fs. The enhancement of the C-O bond breaking of singly charged ethanol at 400 nm and the bond elongation prior to the Coulomb explosion of doubly charged ethanol occurring in the relatively weak light field intensity of 10(12)-10(13) W cm(2) is interpreted by the efficient light-induced coupling among the electronic states at the shorter wavelength of 400 nm. From the double pulse experiment, in which ethanol is irradiated with a pair of short pulses (<80 fs), the most efficient coupling occurs at Deltat=160 fs that is much earlier than Deltat=250 at 800 nm, where Deltat denotes the temporal separation of the two pulses, indicating that the nonadiabatic field-induced potential crossings of singly charged ethanol occurs much earlier at 400 nm than at 800 nm.

Suppression of decomposition of aniline cation in intense laser fields by cluster formation with NH3 and H2O

Abstract Using a tandem-type time-of-flight (TOF) mass spectrometer, the mass-selected aniline (AN)–water cluster cations, [AN(H 2 O) n ] + ( n =1, 2), were irradiated with the intense femtosecond (fs) laser fields ( λ ∼395xa0nm, I ∼5×10 15 xa0W/cm 2 , Δ t ∼50xa0fs). By the cluster formation with H 2 O, decomposition of AN + , AN + →C 5 H 6 + +HNC, induced by the intense fs laser fields was significantly suppressed. From the observation that the suppression of the decomposition of AN + occurs from [AN(H 2 O) n ] + more efficiently than from [AN(NH 3 ) n ] + ( n =1, 2), the effect of the cluster formation on the decomposition process was discussed.

Resonance-state selective photodissociation of OCS (2 1Σ+): Rotational and vibrational distributions of CO fragments

The rotational and vibrational state distributions of the CO fragments produced through the photodissociation of OCS in the vacuum ultraviolet (VUV) region (150–155 nm), OCS (2u200a1Σ+)→COu200a(Xu200a1Σ+)+S(1S), are derived for the three lowest quasi-bound vibrational resonances (v*=0−2) in the 2u200a1Σ+ state. The rotational state distributions of the CO fragments in the vCO=0 and 1 vibrational states are determined, respectively, by the analysis of the rotational structures in the laser-induced fluorescence (LIF) spectra of the A1Π–Xu200a1Σ+(0,0) and (1,1) transitions of CO. The rotational temperatures of CO in the vCO=0 state are low (∼100 K) for all the three resonances, while those in the vCO=1 state are substantially higher, i.e., 2210, 940, and 810 K for v*=0, 1, and 2, respectively. The vibrational state distributions of CO are derived from the Doppler spectroscopy of the counterpart S(1S) fragments. From the analysis of the observed Doppler profiles, it is found for all the three lowest vibrational resonances of OCS...

Controlling the dissociative ionization of ethanol with 800 and 400 nm two-color femtosecond laser pulses.

Ethanol molecules were irradiated with a pair of temporally overlapping ultrashort intense laser pulses (10(13)-10(14) Wcm(2)) with different colors of 400 and 800 nm, and the dissociative ionization processes have been investigated. The yield ratio of the C-O bond breaking with respect to the C-C bond breaking was varied in the range of 0.17-0.53 sensitively depending on the delay time between the two laser pulses, and the absolute value of the yield of the C-O bond breaking was found to be increased largely when the Fourier-transform limited 800 nm laser pulse overlaps the stretched 400 nm laser pulse, demonstrating an advantage of the two-color intense laser fields in controlling chemical bond breaking processes.