Research paperIdentification of the cationic excited state of cyclopentanone via time-resolved Ion yield measurements
Graphical abstract
Introduction
When gas-phase molecules are subjected to a laser field with pulse duration of tens of femtoseconds and peak intensity over 1013 W/cm2, they will undergo ultrafast photoionization, which is the first and fundamental process for the outcome of strong laser-molecule interaction [1]. One particular issue in the ultrafast photoionization of polyatomic molecules is their fragmentation patterns, which are mainly produced via dissociation of parent ions. It is interesting to be observed that, for some molecules, photoionization produces little fragments in femtosecond laser fields, which is quite unlike that in nano-second laser fields where fragmentation is dominant in multiphoton ionization [2]. Various studies have shown that resonance in cationic electronic states plays important role in the observed fragmentation pattern in ultrafast photoionization of molecules, that is, if the photon energy is resonant with cationic excited electronic states, extensive fragmentation occurs, otherwise, the parent ions are dominant in ultrafast photoionization [3], [4], [5], [6], [7].
Comparing to that of neutral molecules, our knowledge about electronic states of molecular cations is relatively sparse, partly due to the difficulty in producing high-number density of the ground state cations. The fact that fragmentation in molecular ultrafast photoionization depends on the resonances in the cationic states provides us a new alternative method to identify the excited states of the cations and to investigate their ultrafast dynamics. For example, with the usage of femtosecond photoionization–photofragmentation spectroscopy, Ho et al. [8] have probed ultrafast dynamics of the azobenzene cations and have identified photoionization-induced twisting in the cations. More recently, a one-photon ionic resonance in acetophenone have been measured by Bohinski et al., using strong-field, near-infrared tunnel ionization and excitation method with wavelength scanning from 800 to 1500 nm [9]. Weinacht and co-workers have performed a series of studies on the dynamics of vibrational wavepackets of halogenated methane cations in strong-field ionization [10], [11], [12], [13], [14]. Oscillation structure in both the parent and fragment ion signals as a function of pump-probe was revealed, which was attributed to the dynamics of the ground state vibrational wavepackets and the one-photon resonance with the repulsive excited electronic state of the cations.
Here, we focus on the cationic electronic states of cyclopentanone (CPO) based on ultrafast photoionization. As a member in the family of unsaturated aliphatic cycloketones that are well known for their flexible structures, CPO has been attracted numerous studies. The spectroscopy and dynamics of neutral CPO have been extensively investigated [15], [16], [17], [18], [19], [20], [21], [22], [23]. The spectra and photodissociation dynamics of neutral CPO were proposed based on the results of these studies. Several studies have been performed on the strong-field photoionization of CPO [24], [25], along with other cycloketones [7] and methyl-substituted cyclopentanone [26]. The results indicated that fragmentation was enhanced with increasing laser intensity and molecular size. Wang et al. [25] have discussed dissociation pattern from the ground state surface of CPO cations based on RRKM (Rice–Ramsperger–Kassel–Marcus) theory and ab initio calculations. Wu et al. [7] have measured the mass spectra of several cycloketones in a 788 or 394 nm laser field with 90-fs pulse duration and intensity from 5 × 1013 W/cm2 to 2 × 1014 W/cm2. Dominant parent ion signal for CPO was observed at 788 nm, while extensive fragmentation was shown in the mass spectra at 394 nm, which was interpreted by the resonance in the cationic electronic states according to the calculated absorption spectrum of CPO+.
In our study, we performed femtosecond time-resolved ion yield measurements on CPO, using strong 800-nm laser pulses as the pump and weak 400-nm laser pulses as the probe. The transients of both parent and fragment ions were obtained by recording the ion yield as a function of the pump-probe delay time. Dependence of the ion yields on the peak intensity of the probe laser was investigated, the results of which demonstrate the one 400-nm photon resonance in the excited electronic state of CPO+. Theoretical calculations indicate that the cationic excited state involved in the pump-probe measurements is the D4 state with 2A symmetry. Possible dissociation mechanism of the D4 state of CPO+ is discussed based on the experimental and theoretical results.
Section snippets
Experimental
The experimental studies on the cationic electronic states of CPO were carried out using the femtosecond pump-probe technique and a linear time-of-flight (ToF) mass spectrometer operated under the Wiley–Mclaren condition. The laser system used in the experiments is a Ti: Sapphire chirped-pulse amplified laser (Libra-USP-HE, Coherent Inc.) with central wavelength of 800 nm, pulse duration of 50 fs, repetition rate of 1 kHz and maximum pulse energy of 4 mJ. The fundamental output of the laser system
Pump-probe transients
The dominant peaks in the TOF mass spectra with irradiation by only the 800-nm laser at ∼6 × 1013 W/cm2 are parent CPO ions (CPO+, m/z = 84), along with small fractions of fragment ions, C2H4+ (m/z = 28) and C4H8+/C3H4O+ (m/z = 56), consistent with the previous study using 90-fs 788-nm laser fields [7]. This prepares the CPO+ source that allows us to investigate the cationic excited electronic states. By employing both the 800-nm and the 400-nm laser pulses, the transients of parent ions CPO+ as well as
Conclusion
In conclusion, we have investigated the cationic excited electronic state of CPO using femtosecond pump-probe method. The formation of the C4H8+/C3H4O+ fragment ions has been observed in the transient, while the transients of the C2H4+ fragment ions as well as the parent ions show constant depletion. The cationic excited state involved in the pump-probe process is determined to be at one 400-nm photon resonance from the ground D0 state, which is assigned as the D4(2A) state of the cations based
Acknowledgments
This work was supported by National Basic Research Program of China (973 Program) (2013CB922200) and National Natural Science Foundation of China (11534004, 11274140, 11574115, 11574114, U1532138).
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