【作者】 杨景；

【导师】 丁大军； 金明星；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2004， 硕士

【摘要】 过去几十年，碘甲烷是一种被广泛研究的分子。我们在脉宽为100fs波长为800nm光强为1014W/cm2左右的激光系统下对其电离解离机制进行了研究，Cm+(m≦4)，In+(n≦6)，CHn+（n=1~3）等离子，还有CH22+产生，更高价的离子由于其质谱峰与O+和H2O+的质谱峰重合而无法辨清（在实验中还可能产生了碘的I7+、I8+）。碘甲烷分子很容易从基态被激发到3Q0，3Q1，1Q1三个激发态，发生解离，产生甲基和碘原子等中性碎片，这个过程只需吸收2个800nm的光子，然而碘甲烷分子被电离则需要吸收更多的800nm的光子，因此碘甲烷可能是先解离然后再被电离。El-Sayed等人在266nm，脉宽为35ps激光系统下对碘甲烷分子进行了研究，证明先电离然后解离是其主要机制。C.Kosmidis等人在1064nm，532nm的ps，ns的激光系统下、P.Graham等人及Castleman等人在790nm左右飞秒激光系统下也分别对碘甲烷分子进行了研究，都认为碘甲烷分子先电离然后解离。 <WP=64>在质谱中，母体离子明显存在，观察到的多电荷的原子离子Cm+(m≦4)和In+(n≦6)都存在复杂峰型。这样现象在P.Graham及Castleman等人的飞秒实验中都被观察到，认为复杂峰型是母体离子库仑爆炸产生的。在低强度的ps和ns实验中这样的峰型也有记录，因此质谱上所看到的复杂峰型，可能是探测器探测到的正向及反向飞向探测器的离子。所以要进一步确认碘甲烷分子在该实验条件下的电离解离机制还需考虑碎片离子的动能分布。我们实验所用光强比其它相关飞秒实验要低很多，为了进一步证明所观察到的复杂峰型的成因，我们通过相同价态的碘离子到达探测器的时间差，计算了母体离子在反应室解离瞬间的动能。随着碘离子价态升高，碎片瞬间动能增大，这证明高价态碘离子不是低价态碘离子的多光子电离得来的，所以他们应该是母体分子的高价离子爆炸得来的，因此在该实验中有库仑爆炸发生。在质谱上我们还观察到CH22+碎片，没有观察到CH2+，CH32+等离子碎片，通过gaussian98软件包，在PC机上用MP2/6-311g*分别对CH2+，CH22+，CH32+构型优化和频率分析，发现CH2+，CH32+不是稳定结构，如果它们存在，其寿命也极短不能被我们的探测器所记录，而CH22+能以（H-C-H）2+直线型结构稳定存在。我们还用更大机组对CH3I2+进行了理论计算，发现这种分子离子在基态不是稳定的，因此其存在还需进一步<WP=65>理论和实验验证。P.Graham等人认为在光强低于大约1014W/cm2时只有CH3I+、I+、CH3+等碎片而没有碘和碳的高价离子，而我们用800nm，光强在1014W/cm2左右的激光也观察到了碘和碳的高价离子。在实验中，我们观察到了碘各种离子飞行时间质谱峰的三峰结构，这与Castleman所报道的峰型不大一样，通过简单理论计算和结合实验条件的计算机模拟，发现三峰结构的中间峰对应于碘离子的零动能峰，利用Keldysh因子，认为这些碘离子来源于碘原子的多光子电离。在整个实验中，我们还观察到了各种碘离子强度的规则布居，认为这与激光束的空间结构是紧密相连的，也与我们在引出场前所加的一个狭缝有些关系。本文还试图对同类小分子在强激光场中的电离解离机制建立一种明晰的分析方案，对激光结构、波长、脉宽、光强以及激光偏振对电离解离机制的影响建立一种规律分析。随着实验条件的不断完善这方面的工作将会不断得到补充和改进。更多还原

【Abstract】 Methyl iodide has been extensively studied in the past. Ionization and dissociation mechanism of methyl iodide induced by 100fs femtosecond laser (I~1014W/cm2) light at 800nm was studied by means of a time-of-flight mass spectrometer. We observed that the mass spectra of CH3I irradiated by ths fs laser consists of the highly charged ions of iodide and carbon . In our work I7+and I8+are possible but they would be difficult to distinguish from the O+ and H2O+ in the mass spectrum. The excited states 3Q0,3Q1and 1Q1,which are optically accessible from the ground state ,are dissociative ,leading to the production of CH3 and I neutral fragments. These states can be reached through absorption of two 800nm photons, while more photons are needed for molecular ionization.The recorded mass spectra, at first glance, could be attributed to fragmentation which is followed by ionization. On the contrary, El-Sayed and coworkers have shown that the multiphoton ionization dissociation is the dominant mechanism in the CH3I, when this molecule is irradiated by picosecond laser pulses(30ps at 266nm).C.Kosmidis and coworkers investigated the molecule also at 1064nm, 532nm with ps , ns pulse duration and proved the El-Sayed’s conclusion right. The results from the P.Graham and Castleman’s study on CH3I also supported this conclusion <WP=67>In the mass spectra ,the parent ion peak was clearly recorded .Multiply charged molecular ions have been also observed and all In+ and Cp+ have spectral peaks which were assigned to the Coulomb explosion of multiply charged molecular ions in P.Graham and Castleman’s respectivel femtosecond experiments. But the peak profiles also presented in C.Kosmidis’ experiments at lower laser intensities with picosecond and nanosecond laser pulses.So we have observed complex ion peak profiles not to be assigned arbitrarily to the the Coulomb explosion of multiply charged parent ions, especial to in our lower intensities experiment than the other correlative fs experiments. In order to have more insight, we have calculated the kinetic energies (Ekin) of the fragments ejected to the backward and forward direction to detector by the difference the time they got to detector. The kinetic energy values of I ions increase as their charge values increase, which implies that the higher charged I ions weren’t derived of the lower charged ones by further excitation ejected from the parent ion. Therefore, the Coulomb explosion mechanism within a multiple charged unstable parent ion should be invoked in our experiments. We observed the peak of the CH22+, not observed the peaks of the CH2+ and CH32+. We optimized their structure and analyzed their vibrational frequencies respectively with MP2/6-3111g* basis set by Gaussian98 on the personal computer. And it is found that the CH2+ and CH32+ aren’t stable structure but the CH22+ is stable. If the CH2+ and CH32+ were produced in the experiment, their lives presented were too short to be recorded by the detector. We analyzed also the CH3I2+ by the bigger basis set and found it is unstable in the electronic ground state, so its disappearance is inexplicable <WP=68>which need to be validated further.P.Graham and co-workers considered that only the fragments CH3I+, I+,CH3+ were produced but not highly charged ions in their work at lower intensities of about 1014W/cm2 .But we observed the highly charged ions of the iodine and carbon at 800nm with about 1014W/cm2. In our experiment we observed three-peak structures of iodide ions which were different from the structure reported by Castleman.By theoretical calculation and computer simulation it is found that the middle peaks are the zero-kinetic-energies-peaks of iodide ions. We considered that these iodide ions come from the multiphoton ionization of the iodide atoms.We also observed that the intensities of iodide ions have orderliness contribution which was considered that depend on the space contribution of the laser intensity and also is relative to the slit that was placed in the front of the traction electr更多还原