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

doi:10.7503/cjcu20180152

Abstract

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

CLC Number:

O643

TrendMD:

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

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

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