Qihe Zhu

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.

Communication: Probing the entrance channels of the X + CH4 → HX + CH3 (X = F, Cl, Br, I) reactions via photodetachment of X−–CH4

The entrance channel potentials of the prototypical polyatomic reaction family X + CH(4) → HX + CH(3) (X = F, Cl, Br, I) are investigated using anion photoelectron spectroscopy and high-level ab initio electronic structure computations. The pre-reactive van der Waals (vdW) wells of these reactions are probed for X = Cl, Br, I by photodetachment spectra of the corresponding X(-)-CH(4) anion complex. For F-CH(4), a spin-orbit splitting (∼1310 cm(-1)) much larger than that of the F atom (404 cm(-1)) was observed, in good agreement with theory. This showed that in the case of the F-CH(4) system the vertical transition from the anion ground state to the neutral potentials accesses a region between the vdW valley and transition state of the early-barrier F + CH(4) reaction. The doublet splittings observed in the other halogen complexes are close to the isolated atomic spin-orbit splittings, also in agreement with theory.

Vibrationally resolved photofragment translational spectroscopy of CH3I from 277 to 304 nm with increasing effect of the hot band.

The photodissociation dynamics of CH(3)I from 277 to 304 nm is studied with our mini-TOF photofragment translational spectrometer. A single laser beam is used for both photodissociation of CH(3)I and REMPI detection of iodine. Many resolved peaks in each photofragment translational spectrum reveal the vibrational states of the CH(3) fragment. There are some extra peaks showing the existence of the hot-band states of CH(3)I. After careful simulation with consideration of the hot-band effect, the distribution of vibrational states of the CH(3) fragment is determined. The fraction σ of photofragments produced from the hot-band CH(3)I varies from 0.07 at 277.38 nm to 0.40 at 304.02 nm in the I* channel and from 0.05 at 277.87 nm to 0.16 at 304.67 nm in the I channel . E(int)/E(avl) of photofragments from ground-state CH(3)I remains at about 0.03 in the I* channel for all four wavelengths, but E(int)/E(avl) decreases from 0.09 at 277.87 nm to 0.06 at 304.67 nm in the I channel . From the ground-state CH(3)I, the quantum yield Φ(I*) is determined to be 0.59 at 277 nm and 0.05 at 304 nm. The curve-crossing probability P(cc) from the hot-band CH(3)I is lower than that from the ground-state CH(3)I. The potential energy at the curve-crossing point is determined to be 32,740 cm(-1).

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.

Formation and photolysis of tantalum sulfide cluster ions

Tantalum sulfide cluster ions (TanS+m, n⩽ 9, m⩽ 30) have been produced by direct laser ablation of a mixture of tantalum and sulfur powders, and have been studied with a tandem time-of-flight mass spectrometer. The tantalum sulfide cluster ions in maximum abundance were TanS+2n+ 7(n= 1–9) detected in the first stage mass spectra. The main dissociation path of the UV-photolysis (248 nm) of tantalum sulfide cluster ions is a sequential S2 loss. For cluster ions with n 3, the photolysis yields Ta3S+4 or Ta4S+6 as the major product. Structures with Ta3S4 and Ta4S6 as the framework were suggested for the large tantalum sulfide cluster ions.

The ultraviolet photolysis of acetyl and propionyl radicals studied by infrared emission spectroscopy

The photodissociation of acetyl and propionyl radicals at 248 and 266 nm has been studied by time-resolved Fourier transform infrared spectroscopy. A vibrationally excited product CO(v⩽8) was observed in the emission spectra. The vibrational temperatures of the nascent CO products were about 7400 K for the acetyl radical and 6930 K for the propionyl radical. The vibrational energy partitioning of the CO fragments fits a soft impulsive model.

Photofragment translational spectroscopy of n-C3H7I and i-C3H7I near 280 and 304 nm.

The photodissociation dynamics of propyl iodides n-C3H7I and i-C3H7I near 280 and 304 nm has been investigated with our mini-TOF photofragment translational spectrometer. When a single laser is applied for both the photodissociation of parent molecules and the REMPI of I atom photofragments, the TOF spectra of photofragments I*(2P1/2) and I (2P3/2) are obtained at four different wavelengths for these two iodides. For n-C3H7I, some small vibrational peaks are partially resolved (with separation of approximately 522 cm-1, corresponding to the RCH2 deformation frequency of the fragment n-C3H7) at 281.73, 279.71, and 304.67 nm. These results show that the RCH2 deformation is mostly excited. For i-C3H7I, we obtain some partially resolved vibrational peaks (with separation of approximately 352 cm-1, corresponding to the HC(CH3)2 out-of-plane bending frequency of the fragment i-C3H7) at 281.73 nm only. For n-C3H7I, the partitioning values of the available energy Eint/Eavl are 0.48 at 281.73 nm and 0.49 at 304.02 nm for the I* channel, and 0.52 at both 279.71 and 304.67 nm for the I channel. These energy partitioning values are comparable with the previous results at different wavelengths in the literature. For i-C3H7I, the Eint/Eavl values are 0.61 at 281.73 nm, 0.65 at 304.02 nm for the I* channel, and 0.62 at 279.71 nm, 0.49 at 304.67 nm for the I channel. The potential-energy-surface crossing and the beta values have also been discussed.