The angular distribution of CH3I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8×1013 W/cm2 is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.
Time-of-flight(TOF) mass spectra of molecular butanone clusters were measured under the irradiation of photons at 355 and 118 nm. Butanone molecular parent ion and several series of butanone cluster fragments such as (CH3COC2H5)nH+, (CH3COC2H5)nC2H5+, (CH3COC2H5)nCH3CO+ and (CH3COC2H5)nC2H5CO+ were observed. Odd-even variation pattern in the intensity of (CH3COC2H5)nCH3CO+ is obvious from n=4 to 8. A connection is es-tablished between the fragment clusters (CH3COC2H5)nCH3CO+ and the neutral clusters (CH3COC2H5)n+1 based on the consideration of butanone cluster dissociation mainly by losing a radical C2H5?. The stability variation of the neutral clusters calculated via the HF-B3LYP method based on DFT is in agreement with the observed phenomena experimentally, indicating (CH3COC2H5)n+1(n+1=5,7) with a ring structure are with higher stability over the adjacent even-number clusters.
SUN Chang-kai HU Zhan YANG Xue JIN Ming-xing HU Wen-cheng DING Da-jun
A train of three equally spaced femtosecond laser pulses is employed to control the photoionization/photodissociation processes of cyclopentanone.With the increase of pulse separation,a strong modulation of product ion yield is observed.More than ten-fold changes of ion yield ratio between different products can be realized.The experimental observations further explain the compositions and formation pathways of peaks in the mass spectra.The controlling mechanisms are also discussed.
