CF3I在238 nm的光解碎片平动能谱研究:CF3光解碎片的振动态分布和势能曲线交叉穿越几率

doi:10.7503/cjcu20180152

摘要/Abstract

摘要：

利用高分辨弱电场加速型光解碎片平动能谱仪, 对CF₃I在238 nm的光解动力学进行了研究. 结果表明, 来自³Q₀平行跃迁的I^*通道[CF₃I(X) $\xrightarrow[\parallel]{hv}$CF₃I(³Q₀)→CF₃(ν₁, ν₂)+ I^*], 信号最强, 并可以分辨出碎片CF₃(ν₁, ν₂) 的振动峰. 通过对峰间距的分析, 可以基本分辨出同时具有CF₃碎片的振动波带CF₃(ν₁, ν₂=0)与CF₃(ν₁, ν₂=1). 其中ν₁=701 cm^-1为对称伸缩振动, ν₂=1086 cm^-1为CF₃伞形振动. 测量了这些振动态的分布及振动波带分配比为∑P(ν₁, 1)/∑P(ν₁, 0)=0.48/0.52. 还测得238 nm处4个光解通道的分支比: 通道1(X→³Q₀→ I^*)为0.664, 通道2(X→³Q₀→¹Q₁→I)为0.084, 通道3(X→¹Q₁→I)为0.178, 通道4(X→¹Q₁→³Q₀→I^*)为0.074. 实验结果表明, ³Q₀→¹Q₁过程中的势能曲线交叉穿越几率P_0→1=0.112, ¹Q₁→³Q₀过程中P_1→0=0.294; I^*通道的各向异性参数β(I^*)=1.70, I通道β(I)=-0.04.

关键词: 光解, 三氟碘甲烷, 光解碎片平动能谱, 振动态激发, 势能曲线交叉穿越几率

Abstract:

The photodissociation dynamics of CF₃I at 238 nm was studied using our high resolution mini-TOF(time of flight) photofragment translational spectrometer. The results indicated the strongest signal comes from the I^* channel of the ³Q₀ parallel transition[i.e., CF₃I(X)→CF₃I(³Q₀)→CF₃(ν)+ I^*], and the vibrational peaks of CF₃ fragments were resolved. From the peak gap, both the overtone bands (ν₁, ν₂=0) and the combination bands(ν₁, ν₂=1) of CF₃ fragments were partially resolved(ν₁=701 cm^-1, the symmetric stretch mode; ν₂=1086 cm^-1, the umbrella mode). At 238 nm, the ratio of the two band series was found with ∑P(ν₁, 1)/∑P(ν₁, 0)=0.48/0.52. The vibrational distribution of CF₃ fragments were also measured. The branching fractions of the four pathways were 0.664 for X→³Q₀→ I^*, 0.084 for X→³Q₀→¹Q₁→I, 0.178 for X→¹Q₁→I and 0.074 for X→¹Q₁→³Q₀→I^*. The experimental curve crossing probability for ³Q₀→¹Q₁ was $P 0 → 1$ =0.112 and that for ¹Q₁→³Q₀ was P_1→0 =0.294. The anisotropy parameter of I^* channel was β(I^*)=1.70, and that of I channel was β(I)=-0.04.

Key words: Photodissociation, Trifluoromethyl iodide, Photofragment translational spectroscopy, Vibrational state excitation, Potential curve crossing probability

中图分类号:

O643

TrendMD:

林丹, 程敏, 杜宜奎, 朱起鹤. CF₃I在238 nm的光解碎片平动能谱研究:CF₃光解碎片的振动态分布和势能曲线交叉穿越几率. 高等学校化学学报, 2018, 39(8): 1713.

LIN Dan, CHENG Min, DU Yikui, ZHU Qihe. Photofragment Translational Spectroscopy of CF₃I at 238 nm:Vibrational Distribution of CF₃ Fragments andCurve Crossing Probability^†. Chem. J. Chinese Universities, 2018, 39(8): 1713.

图/表 11

Fig.1 Five pathways of CF3I photodissociation in the A bandPathway 1: X$\xrightarrow[\parallel]{hv}$ 3Q0$\rightarrow$ CF3+I*; Pathway 2: X$\xrightarrow[\parallel]{hv}$ 3Q0$\xrightarrow{c.c.}$1Q1$\rightarrow$CF3+I; Pathway 3: X $\xrightarrow[\perp]{hv}$1Q1 $\rightarrow$F3+I; Pathway 4: X $\xrightarrow[\perp]{hv}$Q1 3$\xrightarrow{c.c.}$0 C$\rightarrow$3+I*; Pathway 5: X 3$\xrightarrow[\perp]{hv}$1 CF$\rightarrow$+I

Fig.2 Schematic diagram of experimental apparatus

Fig.3 TOF spectrum of I+ from CF3I(X) $\xrightarrow[\parallel]{hv}$CF3I(3Q0)→CF3(ν1, ν2)+I*(pathway 1) at 238 nmα =0°, for 3Q0 parallel transition; 304.02 nm for I* REMPI; weak accelerated field E=0.72 V/cm.

Fig.4 TOF spectrum of I+(A) and PTS of I*(B) from CF3I (X) $\xrightarrow[\parallel]{hv}$CF3I(3Q0)→CF3(ν1, ν2)+I* (pathway 1) at 238 nmThe open circles are the experimental data of this work. In panel (A), the parallel dash line shows the ground noise removed and the perpendicular dash line represents the initial point of signal area. The solid and dash lines in panel (B) are the simulated spectra.

Table 1 Vibrational distribution of CF3I (X)→CF3I(3Q0)→CF3(ν1, ν2)+I*(pathway 1) at 238 nm

CF₃(ν₁, ν₂)
(0, 0)	(1, 0)		(2, 0)		(3, 0)		(4, 0)	(5, 0)	(6, 0)		(7, 0)		(8, 0)		(9, 0)		(10, 0)
0.008	0.005		0.013		0.034		0.065	0.083	0.10		0.098		0.060		0.030		0.014
(0, 1)		(1, 1)		(2, 1)		(3, 1)		(4, 1)		(5, 1)		(6, 1)		(7, 1)		(8, 1)
0.011		0.030		0.056		0.062		0.092		0.095		0.080		0.034		0.024

Table 2 Energy partition in CF3I (X)→CF3I(3Q0)→CF3(ν1,ν2)+I*(pathway 1) at different wavelengths

λ/nm	277^[4]	266^[4]	248^[4]	238
E_avl/eV	1.20	1.38	1.73	1.94
E_T/eV	1.07	1.18	1.35	1.42
E_int/eV	0.13	0.20	0.38	0.52
E_int/E_avl	0.108	0.145	0.220	0.266
∑P(ν₁, 1)/∑P(ν₁, 0)	0.28/0.72	0.38/0.62	0.41/0.59	0.48/0.52

Fig.5 TOF spectrum of I+ from CF3I(X) $\xrightarrow[\parallel]{hv}$CF3(3Q0) $\xrightarrow{c.c.}$CF3(1Q1)→ CF3 (ν1, ν2)+I(pathway 2) at 238 nmα=0°; weak accelerated field E=0.12 V/cm.

Fig.6 TOF spectrum of I+ from CF3I(X) $\xrightarrow[\perp]{hv}$CF3(1Q1)→CF3 (ν1, ν2)+I(pathway 3) at 238 nmα =90°; weak accelerated field E=0.12 V/cm.

Fig.7 TOF spectrum of I+ from CF3I(X) $\xrightarrow[\perp]{hv}$CF3(1Q1) $\xrightarrow{c.c.}$CF3(3Q0)→ CF3 (ν1, ν2)+I*(pathway 4) at 238 nmα=90°; weak accelerated field E=0.72 V/cm.

Table 3 Branching fractions of the four pathways

Transition type	Dissociation pathway	Branching fraction
Parallel	X→³Q₀→ I^*	0.664
	X→³Q₀→¹Q₁→I	0.084
Perpendicular	X→¹Q₁→I	0.178
	X→¹Q₁→³Q₀→I^*	0.074

Table 4 Anisotropy parameters and the curve-crossing probability Pc.c. for different dissociation wavelengths in the A absorption band

λ/nm	β(I^*)	β(I)	P_c.c.
λ/nm	β(I^*)	β(I)	³Q₀→¹Q₁	¹Q₁→³Q₀
238	1.70	-0.04	0.112	0.294
248^[14]	<1.75	-0.20	0.04	0.60
266^[4]	1.86	1.03	—	—
277^[4]	1.86	0.98	0.09	—
277^[5]	1.83	0.96	0.09	—

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CF₃I在238 nm的光解碎片平动能谱研究:CF₃光解碎片的振动态分布和势能曲线交叉穿越几率

Photofragment Translational Spectroscopy of CF₃I at 238 nm:Vibrational Distribution of CF₃ Fragments andCurve Crossing Probability^†

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