Fanao Kong

Photodissociation of formic acid

The photodissociation of formic acid has been studied experimentally and theoretically. Ab initio calculations were performed to study the dissociative profiles of five reaction channels on the S0, S1, and T1 potential energy surfaces. The vibrationally excited nascent products were detected using a time-resolved Fourier transform infrared spectrometer after laser photolysis at 248 or 193 nm. In the 248 nm photolysis, the HCOOH molecule was first excited to the S1 state, but it was found that the dissociation takes place on the S0 surface after internal conversion. The products of the vibrationally excited CO, CO2(v3) and H2O(v1) were detected. During the dissociation process the vibrationally energized molecule is geometrically memorized and dynamically controlled, with the yield preference of CO and H2O over that of CO2 and H2. The ratio of CO(v⩾1)/CO2(v⩾1) is estimated as <7.5. Vibrationally excited CO (v) and CO2(v3) are also found in the 193 nm photolysis but the CO/CO2 ratio increases to 11. Most of...

Explosive photodissociation of methane induced by ultrafast intense laser

A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2 x 10(14) W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n = 3-->2), CH (A 2Delta, B 2Sigma-, and C 2Sigma+-->X 2Pi), or C2 (d 3Pi g-->a 3Pi u) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A-->X) emission of methane on a log-log scale has a slope of 10 +/- 1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation.

The production and photodissociation of iron–sulfur cluster ions

Iron–sulfur cluster ions FenSm+ (n=1–13, m=1–13) were produced by direct laser ablation on a solid sample containing a mixture of iron and sulfur powder. UV photodissociation of the cluster ions was studied with a tandem time‐of‐flight mass spectrometer. It was found that all the cluster ions with compositions of m=n, m=n−1, or m=n+5 were relatively more abundant, indicating that they were stable. The photodissociation results of the FenSm+ ions showed that, for parent ions with n≪m, the main channels were sequential losses of neutral S atoms until n∼m, while for parent ions with n∼m, the main product ions had compositions of smaller m=n or m=n−1. From these experimental results, it is proposed that the FenSn+ cluster ions might have structures similar to those of the FenS*n cores in iron–sulfur proteins, while the FenSm+ (m≳n) cluster ions could be considered to have structures with the FenSn+ cores surrounded by some peripheral S atoms.

The ultrafast intramolecular dynamics of phthalocyanine and porphyrin derivatives

The internal conversion and intramolecular vibrational relaxation processes of nitro‐tri‐tert‐butylphthalocyanine, tetra‐phenylporphyrin (TPP), and tetra‐tert‐butylphthalocyanine (BuPc) in chloroform solution were investigated with an ultrafast time‐resolved fluorescence depletion method. A regular fluorescence depletion was observed, indicating that the vibrational relaxation in the S1 state takes a few hundred femtoseconds to several picoseconds. For TPP and BuPc, an additional sharp dip superposes on the regular depletion. It is explained by an ultrafast internal conversion process from the S2 state to the S1 state with a time of a few tens of femtoseconds.

Vibrational relaxation of dye molecules in solution studied by femtosecond time-resolved stimulated emission pumping fluorescence depletion

Abstract A new method, femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD), has been developed to study the vibrational relaxation of electronic excited states of molecules. Two relaxation rates of dye molecules in different solvents have been observed: (i) the intramolecular redistribution of energy, with a short time constant of less than 500 fs; (ii) the subsequent cooling of the vibrationally hot molecules on the picosecond time scale. The rates of these processes strongly depend on the solvent. This may result from a solvent-induced structural modification of the dye molecules.

Direct observation of super-excited states in methane created by a femtosecond intense laser field

We report, for the first time, a direct observation of the super-excited states of CH4 in femtosecond intense laser fields using a pump (800 nm)–probe (1338 nm) technique. An unambiguous depletion of the CH (A 2 � → X 2 �) fluorescence signal as a function of the delay time is attributed to the de-excitation of the super-excited states by the probe laser pulse. The lifetime of the super-excited state is measured to be about 160 fs. (Some figures in this article are in colour only in the electronic version)

The gaseous reaction of vinyl radical with oxygen

The gaseous reaction of vinyl radical with oxygen has been experimentally investigated. C2H3 radical was produced by laser photolysis of C2H3Br at 248 nm. The vibrationally excited products of the reaction were detected by time-resolved Fourier transform infrared emission spectroscopy. H2CO(ν1), HCO(ν1,ν3), and CO2(ν3) are ascertained as the main emitters. The most favorable product channel is HCO and H2CO. The reaction channel leading to CO2+CH3 has been found for the first time. The minor reactions leading to C2H2+HO2, C2H3O+O, and C2H2O2+H may also occur. A secondary reaction product of CO is observed, which is generated from the primary reaction product HCO. Combining theoretical analysis with the present experimental results, the reaction pathways are clarified. The results are of importance for understanding the combustion processes of hydrocarbon.

Dissociation of acetaldehyde in intense laser field: Coulomb explosion or field-assisted dissociation?

Dissociation of acetaldehyde in moderate strong laser field of 1013–1014 W/cm2 was investigated. Singly charged parent ion CH3CHO+ and fragmental ions CH3+, CHO+, C2H4+, O+, CH2CHO+, and H+ were produced by 800 nm laser of 100 fs pulse duration and recorded by time-of-flight mass spectrometer. The CH3+ fragment further dissociated to CH2+, CH+, and C+ ions at higher intensity. Ab initio calculated results show that the singly-, doubly-, and triply charged parent ions are stable. So, the dissociation mechanism was not due to Coulomb explosion of multicharged ion. A field-assisted dissociation (FAD) theory, which assumes that only one bond undergoes dissociation while the rest of the molecular geometry stays unchanged, was employed to treat the dissociation dynamics. Accordingly, the dressed potential energy surfaces of the ground state for the parent and the fragment ions were calculated. Corresponding quasiclassical trajectory calculations show that the bond ruptures take place in the order of C–C, C–O, a...

The vibrational quenching of NO (v = 1 – 11) by N2O studied by time-resolved Fourier transform infrared emission spectroscopy

Abstract The vibrational quenching of NO(v = 1 – 11) by N2O in gas phase was studied by means of the time-resolved Fourier transform infrared emission spectroscopy. NO(v) was formed from the reaction of N2O and O (1D) which was generated by laser photolysis of N2O at 193 nm. Twenty IR spectra of n×30 μs delay were recorded for the time evolution of the vibrational population of NO (v = 1 – 11). The quenching rate constants kv were obtained. Using SSH theory to analyze the values of kν, it was found that they fitted the mechanism of two relaxation processes transferring the energy to ν1 and ν1 + ν2 modes of N2O.

The formation, photodissociation, and bond structure of cobalt–sulfur cluster ions

The formation and photodissociation of cobalt–sulfur cluster ions (ConS+m) produced by laser ablation on a tablet of well‐mixed cobalt and sulfur powder were studied with a home‐built tandem time‐of‐flight (TOF) mass spectrometer. In the mass spectrum, there are many intense peaks of more stable cluster ions ConS+m signified as follows: n=2–5,7, m=n−1; n=6,8–11, m=n−2; n=12–13, m=n−3; n=14–16, m=n−4. The photolysis of the mass‐selected cluster ions was performed with a 248 nm excimer laser. The dissociation patterns support the above composition results. Previous theories about clusters do not explain the experimental results. A relationship between the electron number and orbital number of more stable clusters was recognized. Ab initio calculations were performed on two small cluster ions to determine stable geometries.