反應(yīng)物分子初始振動(dòng)激發(fā)對(duì)H+CH+→C++H2反應(yīng)的影響?

唐曉平 和小虎周燦華 楊陽

1)(中國(guó)科學(xué)院大連化學(xué)物理研究所,分子反應(yīng)動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室,大連 116023)

2)(太原科技大學(xué)應(yīng)用科學(xué)學(xué)院物理系,太原 030024)

3)(中國(guó)科學(xué)院大連化學(xué)物理研究所,中國(guó)科學(xué)院化學(xué)激光重點(diǎn)實(shí)驗(yàn)室,大連 116023)

反應(yīng)物分子初始振動(dòng)激發(fā)對(duì)H+CH+→C++H2反應(yīng)的影響?

唐曉平1)和小虎2)?周燦華3)楊陽1)

1)(中國(guó)科學(xué)院大連化學(xué)物理研究所,分子反應(yīng)動(dòng)力學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室,大連 116023)

2)(太原科技大學(xué)應(yīng)用科學(xué)學(xué)院物理系,太原 030024)

3)(中國(guó)科學(xué)院大連化學(xué)物理研究所,中國(guó)科學(xué)院化學(xué)激光重點(diǎn)實(shí)驗(yàn)室,大連 116023)

(2017年3月2日收到;2017年4月14日收到修改稿)

在CH+2體系的電子基態(tài)勢(shì)能面上運(yùn)用準(zhǔn)經(jīng)典軌線方法,研究了當(dāng)碰撞能E=500 meV時(shí),反應(yīng)物分子的振動(dòng)激發(fā)對(duì)H(2S)+CH+(X1Σ+)→C+(2P)+H2(X1Σ+g)反應(yīng)的反應(yīng)概率、反應(yīng)截面和立體動(dòng)力學(xué)性質(zhì)的影響.分別計(jì)算了兩矢量相關(guān)k-j′的P(θr)分布,三矢量相關(guān)k-k′-j′的P(φr)分布以及反應(yīng)產(chǎn)物的四個(gè)極化微分截面.結(jié)果表明,產(chǎn)物分子轉(zhuǎn)動(dòng)角動(dòng)量不僅在Y軸方向有取向效應(yīng),還定于Y軸的正方向.并且發(fā)現(xiàn),隨著振動(dòng)量子數(shù)的增加,對(duì)反應(yīng)體系產(chǎn)物分布的影響就越明顯.

立體動(dòng)力學(xué),準(zhǔn)經(jīng)典軌線方法,矢量相關(guān)

1 引 言

H原子[1]與CH+碰撞時(shí)發(fā)生的抽取反應(yīng)在天體物理中占有很重要的地位,反應(yīng)的產(chǎn)物為C+和H2.而在該反應(yīng)式 H+CH+→ C++H2中,生成產(chǎn)物之前需先形成短壽命的中間絡(luò)合物CH+2,再由CH+2快速分離成C+和H2.自1941年CH+被首次識(shí)別以來[2],因其含量過于豐富一直備受關(guān)注.多年來,有關(guān)CH+2體系的研究頗受重視[2?11].例如:1986年Ervin 和Armentrout[10]報(bào)道了C++H2(D2,HD)→H(D)+CH+(CD+)反應(yīng)的反應(yīng)截面以及動(dòng)力學(xué)同位素效應(yīng),主要介紹了從閾值到15 eV的碰撞區(qū)間,反應(yīng)截面在給定的范圍內(nèi)會(huì)快速增長(zhǎng),并且在高能區(qū)時(shí),還出現(xiàn)了同位素效應(yīng),但該效應(yīng)由于振動(dòng)零點(diǎn)能的影響呈現(xiàn)出了不同的結(jié)果.在實(shí)驗(yàn)方面,Plasil等[11]報(bào)道了低溫條件下CH+與H原子在碰撞反應(yīng)中,當(dāng)溫度為60 K時(shí)速率常數(shù)達(dá)到一個(gè)最大值,而在低于該溫度時(shí),實(shí)驗(yàn)值與計(jì)算的結(jié)果正好相反.最近,Werfelli等[4]基于新的勢(shì)能面,運(yùn)用非含時(shí)量子散射法研究了低溫條件下H+CH+→C++H2反應(yīng)的速率常數(shù).結(jié)果表明,溫度在50—800 K的范圍內(nèi)理論值與實(shí)驗(yàn)結(jié)果符合得很好,而當(dāng)溫度低于50 K時(shí)理論值遠(yuǎn)遠(yuǎn)大于實(shí)驗(yàn)值.Han等[12?15]運(yùn)用成熟的準(zhǔn)經(jīng)典軌線法,在吸引、混合、排斥勢(shì)能面上研究了產(chǎn)物雙分子反應(yīng)的轉(zhuǎn)動(dòng)極化.為了獲得一個(gè)完整的理論動(dòng)態(tài)反應(yīng),不僅要研究它的標(biāo)量性質(zhì),也要重視它的矢量性質(zhì)[16].最近,我們小組在研究碰撞能對(duì)CH+2體系的立體動(dòng)力學(xué)性質(zhì)的影響中發(fā)現(xiàn)[17],該反應(yīng)的兩矢量、三矢量分布在低能區(qū)時(shí)隨碰撞能的增加而減弱,但在高能區(qū)時(shí)隨碰撞能的增大該類分布卻明顯增強(qiáng).由此可見,碰撞能對(duì)CH+2體系的影響很明顯.迄今為止,尚未見有關(guān)反應(yīng)物在不同的振動(dòng)量子數(shù)下對(duì)CH+2體系的立體動(dòng)力學(xué)性質(zhì)的影響的研究.所以,本文工作主要是基于最新的勢(shì)能面[18]運(yùn)用準(zhǔn)經(jīng)典軌線法[19]來計(jì)算H+CH+→C++H2反應(yīng).

2 準(zhǔn)經(jīng)典軌線的方法理論

2.1 坐標(biāo)系的建立

計(jì)算中采取質(zhì)心坐標(biāo)系來描述反應(yīng)物的相對(duì)速度k和產(chǎn)物相對(duì)速度k′以及產(chǎn)物轉(zhuǎn)動(dòng)角動(dòng)量j′的分布,如圖1所示.坐標(biāo)系的Z軸正方向平行于反應(yīng)物相對(duì)速度矢量k的方向,Y軸垂直于含有反應(yīng)物相對(duì)速度矢量k和產(chǎn)物相對(duì)速度矢量k′的X-Z平面,該平面也被稱為散射平面.k和k′的夾角θt為散射角,分別用θr和φr來表示產(chǎn)物轉(zhuǎn)動(dòng)角動(dòng)量j′的極角和方位角.

圖1 描述k,k′和j′分布的質(zhì)心坐標(biāo)系Fig.1.The center-of-mass coordinate system used to describe the k,k′and j′correlations.

2.2 產(chǎn)物分子角動(dòng)量的轉(zhuǎn)動(dòng)極化

為了對(duì)動(dòng)力學(xué)信息更加直觀地描述,我們計(jì)算了兩矢量相關(guān)和三矢量相關(guān)的分布函數(shù).通常產(chǎn)物分子k-j′兩矢量相關(guān)的分布函數(shù)P(θr)可以用Legendre多項(xiàng)式[20?22]展開:稱為定向系數(shù)(奇數(shù))或者稱為取向系數(shù)(偶數(shù)),尖括號(hào)表示求平均值.

描述k,k′和j′三矢量相關(guān)二面角分布函數(shù)可以用P(φr)函數(shù)來表征,該函數(shù)可以用Fourier級(jí)數(shù)展開[23?25],即

其中,αn和bn分別為

在計(jì)算過程中,為保證P(φr)收斂則取n=24即可.

定義產(chǎn)物轉(zhuǎn)動(dòng)角動(dòng)量j′的空間分布函數(shù)P(θr,φr)可以表示為

其中,Ckq(θr,φr)是修正的球諧函數(shù),計(jì)算過程中P(θr,φr)展開到k=7便呈現(xiàn)出良好的收斂結(jié)果.

聯(lián)系k,k′和j′三矢量角分布函數(shù)可以寫為

在很多光誘導(dǎo)的雙分子反應(yīng)實(shí)驗(yàn)中,人們只對(duì)k=0和k=2的極化微分截面感興趣,因此只計(jì)算了(2π/σ)(dσ00/dωt),(2π/σ)(dσ20/dωt),(2π/σ)(dσ22+/dωt),(2π/σ)(dσ22?/dωt)四個(gè)極化微分反應(yīng)截面.為了保證收斂,在計(jì)算中極化微分反應(yīng)截面展開到k=7.

2.3 勢(shì)能面

采用準(zhǔn)經(jīng)典軌線法并基于最新的[18]勢(shì)能面,研究了反應(yīng)物分子的振動(dòng)量子數(shù)對(duì)H(2S)+CH+(X1Σ+) → C+(2P)+H2(X1Σ+g)反應(yīng)的立體動(dòng)力學(xué)性質(zhì)的影響.該反應(yīng)放熱量為0.496 eV(能量單位),且勢(shì)阱深度超過4 eV.實(shí)驗(yàn)值[25]顯示H2的解離能為4.751 eV,CH+的解離能[2]為4.255 eV,而在Li等[18]研究的勢(shì)能面上H2和CH+的解離能分別為4.748 eV,4.255 eV.很容易看出,Li等[18]研究的勢(shì)能面與實(shí)驗(yàn)結(jié)果非常接近,從而得出該勢(shì)能面的精確度很高.

2.4 準(zhǔn)經(jīng)典軌線計(jì)算

在最新的勢(shì)能面[18]上,采用準(zhǔn)經(jīng)典軌線方法進(jìn)行計(jì)算.計(jì)算時(shí),將反應(yīng)物分子振動(dòng)量子數(shù)分別取為v=0,1,3,5;轉(zhuǎn)動(dòng)量子數(shù)取為j=0,碰撞能為E=500 meV,積分步長(zhǎng)為0.1 fs,總的軌線條數(shù)為50000;H原子和CH+離子質(zhì)心間的初始距離取為15 ?,在它附近的勢(shì)能值很小,約為10?10eV的能量級(jí),因此相互作用力非常弱,可將其忽略.反應(yīng)的最大碰撞參數(shù)bmax的確定方法是:先運(yùn)行5000條軌線,初步確定bmax的范圍,再選擇用50000條軌線,逐漸增加b使其反應(yīng)的軌線條數(shù)不再增加即可[21].

3 結(jié)果與分析

圖2描述了H+CH+→C++H2(v=0,1,3,5,j=0)的反應(yīng)概率(P00-all)隨初始振動(dòng)量子數(shù)的變化情況.在計(jì)算過程中,碰撞能E=500 meV,將bmax調(diào)為零可以得到反應(yīng)的軌線條數(shù),然后將其與總的軌線條數(shù)作比,可以求出每個(gè)振動(dòng)量子數(shù)所對(duì)應(yīng)的反應(yīng)概率.從圖形的整體上看,H和CH+發(fā)生正碰時(shí)的反應(yīng)概率在v=1時(shí)最大,但隨著激發(fā)態(tài)振動(dòng)量子數(shù)的增加反應(yīng)概率呈下降趨勢(shì).

圖3描述了H+CH+→C++H2(v=0,1,3,5,j=0)的反應(yīng)截面.我們知道,反應(yīng)截面對(duì)于一個(gè)產(chǎn)物通道α是這樣被定義的:這里Nα表示最大碰撞參數(shù)bmax的值滿足反應(yīng)式收斂時(shí)所對(duì)應(yīng)的反應(yīng)軌線條數(shù),N表示總的軌線條數(shù).從整體上看圖3,會(huì)發(fā)現(xiàn)v=0激發(fā)到v=5能級(jí)時(shí),反應(yīng)截面由17.02 ?2減小到14.33 ?2.可以看出,該放能反應(yīng)的反應(yīng)截面最大值和最小值之間的差異很大,也就是說振動(dòng)量子數(shù)對(duì)此引起的變化很明顯.

圖2 不同的振動(dòng)態(tài)(v=0,1,3,5)下H+CH+→C++H2反應(yīng)的反應(yīng)概率(P00-all)Fig.2. Reaction probability of the H+CH+ →C++H2reaction at di ff erent vibrational state(v=0,1,3,5).

圖3 不同的振動(dòng)態(tài)(v=0,1,3,5)下H+CH+→C++H2反應(yīng)的橫截面Fig.3. Integral reaction cross section of the H+CH+→C++H2reaction at di ff erent vibrational state(v=0,1,3,5).

圖4為H+CH+→C++H2反應(yīng)的k-j′兩矢量相關(guān)函數(shù)P(θr)的分布情況.由圖4可知,函數(shù)P(θr)在θr=90?時(shí)有一個(gè)明顯的峰值,并且關(guān)于θr=90?呈軸對(duì)稱分布.這說明,產(chǎn)物轉(zhuǎn)動(dòng)角動(dòng)量分布傾向于垂直矢量k的方向上.當(dāng)反應(yīng)物分子處于激發(fā)態(tài)時(shí),峰值明顯變大而寬度基本沒有改變,這相比于基態(tài)取向效應(yīng)大大增強(qiáng),且隨著激發(fā)態(tài)振動(dòng)量子數(shù)的增加峰值的升高幅度更加明顯.對(duì)CH+2體系,通過圖4可以發(fā)現(xiàn)當(dāng)v=0增加到v=5時(shí),在θr=90?處對(duì)應(yīng)的P(θr)的峰值最低為0.56,最高達(dá)到0.7左右,增加了0.14.而在這之前本課題組研究該體系的碰撞能對(duì)此反應(yīng)的影響發(fā)現(xiàn)[17],碰撞能由1 meV增加到1000 meV時(shí),θr=90?對(duì)應(yīng)的P(θr)的峰值最小值為0.56,最大值為0.65,大約變化了0.09.通過比較我們不難發(fā)現(xiàn),振動(dòng)量子數(shù)對(duì)兩矢量分布的影響比碰撞能稍大一點(diǎn).

圖4 (網(wǎng)刊彩色)不同的振動(dòng)態(tài)(v=0,1,3,5)下H+CH+→C++H2反應(yīng)的P(θr)分布Fig.4.(color online)Angular distribution of P(θr)the H+CH+→C++H2reaction at di ff erent vibrational state(v=0,1,3,5).

圖5為H+CH+→ C++H2反應(yīng)的k-k′-j′三矢量相關(guān)函數(shù)P(φr)的分布情況.如圖5所示,P(φr)分布關(guān)于φr=180?不對(duì)稱,反映了產(chǎn)物轉(zhuǎn)動(dòng)角動(dòng)量的強(qiáng)烈極化效應(yīng).在φr=90?時(shí),基態(tài)情況(v=0)下有峰值出現(xiàn),但峰值不是很明顯.而在激發(fā)態(tài)(v=1,3,5)情況下,此處的峰值隨振動(dòng)量子的增加逐漸增大,當(dāng)振動(dòng)量子數(shù)v=5時(shí)峰值達(dá)到最大.在φr=270?處,基態(tài)時(shí)峰值基本處于平緩狀態(tài),而激發(fā)態(tài)處的峰值高于基態(tài),但隨著振動(dòng)量子數(shù)的增加峰值無規(guī)律變化,且在v=1處峰值最大.總體對(duì)比后發(fā)現(xiàn),在φr=90?處的峰值要高于φr=270?處的峰值,這也說明了產(chǎn)物分子轉(zhuǎn)動(dòng)角動(dòng)量j′不僅沿著Y軸有取向分布,還定向于Y軸的正方向并且定向效應(yīng)非常強(qiáng).這種既有定向又有取向的分布情況可以用三原子反應(yīng)的排斥模型[26]來解釋.

圖6為H+CH+→C++H2反應(yīng)的產(chǎn)物轉(zhuǎn)動(dòng)角動(dòng)量的空間分布函數(shù)P(θr,φr).圖6(a)—(d)分別對(duì)應(yīng)振動(dòng)激發(fā)(v=0,1,3,5)的空間分布情況.為了便于比較,我們將圖中的概率分布的顯示范圍全部調(diào)整為0.04—0.20區(qū)間段.從圖6可以看出,θr=90?和φr=270?處P(θr,φr)有明顯大小不同的峰值,四幅圖中θr=90?的峰值都比φr=270?處的峰值高,并且隨著反應(yīng)物初始振動(dòng)量子數(shù)的增加,峰值變得更高,同時(shí)增高的程度也變得更明顯.當(dāng)反應(yīng)物分子處于基態(tài)(v=0)和較低的振動(dòng)激發(fā)態(tài)(v=1)時(shí),如圖6(a)和圖6(b)所示,發(fā)現(xiàn)φr=270?處的峰值非常小,這說明初始反應(yīng)物分子的振動(dòng)激發(fā)對(duì)產(chǎn)物分子的轉(zhuǎn)動(dòng)定向效應(yīng)的影響不大.而圖6(c)和圖6(d)中峰值卻很明顯,此時(shí)的反應(yīng)物振動(dòng)量子數(shù)分別為v=3和v=5.因此,隨著振動(dòng)量子數(shù)的增加該分布也變得越來越集中,這說明產(chǎn)物的轉(zhuǎn)動(dòng)角動(dòng)量的轉(zhuǎn)動(dòng)極化程度隨之增強(qiáng).反應(yīng)產(chǎn)物的空間分布P(θr,φr)與圖4中P(θr)分布和圖5中P(φr)分布結(jié)果完全對(duì)應(yīng).

圖5 (網(wǎng)刊彩色)不同的振動(dòng)態(tài)(v=0,1,3,5)下H+CH+→C++H2反應(yīng)的P(φr)分布Fig.5.(color online)Angular distribution of P(φr)the H+CH+→C++H2reaction at di ff erent vibrational state(v=0,1,3,5).

圖7表示的是不同振動(dòng)激發(fā)(v=0,1,3,5)下,H+CH+→C++H2反應(yīng)的極化微分反應(yīng)截面的分布情況.極化微分反應(yīng)截面是描述k-k′-j′三矢量相關(guān)或者產(chǎn)物的散射方向,與產(chǎn)物轉(zhuǎn)動(dòng)角動(dòng)量的定向和取向無關(guān). 圖7(a)中(2π/σ)(dσ00/dωt)只與反應(yīng)物及其產(chǎn)物的相對(duì)速度矢量有關(guān),我們不難發(fā)現(xiàn)產(chǎn)物分子有明顯的前向和后向散射,且隨振動(dòng)量子數(shù)的增加前向散射和后向散射程度有減弱的趨勢(shì). 極化微分反應(yīng)截面分量(2π/σ)(dσ20/dωt)在圖7(b)中給出,與圖7(a)曲線具有相反的分布趨勢(shì).極化微分反應(yīng)截面(2π/σ)(dσ20/dωt)的分布情況也表明了j′分布取向于垂直k的方向.

圖6 (網(wǎng)刊彩色)不同的振動(dòng)態(tài)下H+CH+→C++H2反應(yīng)的空間分布P(θr,φr) (a)v=0;(b)v=1;(c)v=3;(d)v=5Fig.6.(color online)Spatial distribution of P(θr,φr)of the H+CH+→ C++H2reaction at di ff erent vibrational state:(a)v=0;(b)v=1;(c)v=3;(d)v=5.

圖7 (網(wǎng)刊彩色)不同的振動(dòng)態(tài)(v=0,1,3,5)下H+CH+→ C++H2反應(yīng)的極化微分反應(yīng)截面 (a)(2π/σ)(dσ00/dωt);(b)(2π/σ)(dσ20/dωt);(c)(2π/σ)(dσ21?/dωt);(d)(2π/σ)(dσ22+/dωt)Fig.7.(color online)Four polarization dependent di ff erential cross-sections of the H+CH+→C++H2reaction at di ff erent vibrational state:(a)(2π/σ)(dσ00/dωt);(b)(2π/σ)(dσ20/dωt);(c)(2π/σ)(dσ21?/dωt);(d)(2π/σ)(dσ22+/dωt).

前面的兩個(gè)截面(圖7(a)和圖7(b))均為q=0的情況, 對(duì)于q=0時(shí)如圖7(c)和圖7(d)所示,(2π/σ)(dσ21?/dωt),(2π/σ)(dσ22+/dωt)的極化微分反應(yīng)截面在極端向前和極端向后散射的值為零.因在這些極限散射角的情況k-k′的平面不確定,故q?=0時(shí)的極化微分反應(yīng)截面的值必為零.在圖7(c)中的四個(gè)振動(dòng)能級(jí)下,(2π/σ)(dσ21?/dωt)值有正有負(fù),沒有較強(qiáng)的極化效應(yīng),因此振動(dòng)量子數(shù)對(duì)該分布的影響較小. 從圖7(d)可見(2π/σ)(dσ22+/dωt)在整個(gè)角分布里是負(fù)值,這表明產(chǎn)物更傾向于Y軸方向分布.每個(gè)反應(yīng)都有三個(gè)強(qiáng)極化,三個(gè)極化角分別在30?,90?,和150?附近最為強(qiáng)烈,隨著振動(dòng)能級(jí)的變化,v=1時(shí)取向效應(yīng)最明顯.

4 結(jié) 論

本文采用準(zhǔn)經(jīng)典軌線法,在最新的勢(shì)能面[18]上對(duì)H(2S)+CH+(X1Σ+)→ C+(2P)+H2(X1Σ+g)反應(yīng)的反應(yīng)概率、反應(yīng)截面以及立體動(dòng)力學(xué)性質(zhì)進(jìn)行了研究.結(jié)果表明,反應(yīng)概率和反應(yīng)截面都隨反應(yīng)物的初始振動(dòng)量子數(shù)的增加呈下降趨勢(shì).當(dāng)碰撞能E=500 meV時(shí),理論計(jì)算的兩矢量、三矢量以及空間分布情況均隨著振動(dòng)量子數(shù)的增加,產(chǎn)物的轉(zhuǎn)動(dòng)角動(dòng)量更傾向于Y軸的方向,并且定向于Y軸的正方向.同樣極化微分反應(yīng)截面也隨著振動(dòng)量子數(shù)的變化而變化,文中還描述了四個(gè)極化微分反應(yīng)截面的變化情況,(2π/σ)(dσ00/dωt)隨振動(dòng)量子數(shù)的增加前向散射和后向散射均減弱,(2π/σ)(dσ20/dωt)的分布與(2π/σ)(dσ00/dωt)曲線變化的趨勢(shì)相反,而(2π/σ)(dσ21?/dωt)沒有較強(qiáng)的極化,(2π/σ)(dσ22+/dωt)的分布有三個(gè)強(qiáng)極化,且隨著振動(dòng)量子數(shù)的增加三個(gè)強(qiáng)極化的程度也有明顯的變化.綜上所述,該反應(yīng)的立體動(dòng)力學(xué)性質(zhì)對(duì)振動(dòng)量子數(shù)有很強(qiáng)的依賴性.

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PACS:34.50.Lf,31.15.xv,87.15.H–DOI:10.7498/aps.66.123401

E ff ect of reagent vibrational excitation on reaction of H+CH+→ C++H2?

Tang Xiao-Ping1)He Xiao-Hu2)?Zhou Can-Hua3)Yang Yang1)

1)(State Key Laboratory of Molecular Reaction Dynamics,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian 116023,China)
2)(School of Applied Science,Taiyuan University of Science and Technology,Taiyuan 030024,China)
3)(Key Laboratory of Chemical Lasers,Dalian Institute of Chemical Physics,Chinese Academy of Science,Dalian 116023,China)

2 March 2017;revised manuscript

14 April 2017)

The e ff ect of reagent vibrational excitation on the stereodynamical properties of H(2S)+CH+(X1Σ+) →C+(2P)+H2(X1Σ+g)reaction is investigated by quasi-classical trajectory method on a globally smooth ab initio potential surface of the 2A′state at a collision energy of 500 meV.The reaction probability and the reaction cross-section are also studied.In the calculation,the vibrational levels of the reactant molecules are taken as v=0,1,3,5 and j=0,respectively,where v is the vibrational quantum number and j is the rotational quantum number.The calculation results show that the reaction probability reaches a maximum when v=1,and then decreases with the vibrational quantum number increasing.The integral cross-section decreases sharply with the increase of vibrational quantum number.The potential distribution P(θr),the dihedral angle distribution P(φr),and the polarization-dependent generalized di ff erential cross sections are calculated.P(θr)represents the relation between the reagent relative velocity k and the product rotational angular momentum j′.P(φr)describes the correlation of k-k′-j′,in which k′is the product reagent relative velocity.The peak of P(θr)is at θr=90?and symmetric with respect to 90?,which shows that the product rotational angular momentum vector is strongly aligned along the direction perpendicular to the relative velocity direction.The peak of P(θr)distribution becomes increasingly obvious with the increase of the rotational quantum number.The dihedral angle distribution P(φr)tends to be asymmetric with respect to the k-k′scattering plane(or about φr=180?),directly re fl ecting the strong polarization of the product angular momentum for the title reaction.Each curve has two evident peaks at about φr=90?and φr=270?,but the two peak intensities are obviously di ff erent,which suggests that j′is not only aligned,but also strongly orientated along the Y-axis of the center-of-mass frame.The peak at φr=90?is apparently stronger than that at φr=270?,which indicates that j′tends to be oriented along the positive direction of Y-axis.In order to validate more information,we also plot the angular momentum polarization in the forms of polar plots θrand φr.The distribution of P(θr,φr)is well consistent with the distribution P(θr)and also the distribution P(φr)of the products at di ff erent vibrational quantum states.In addition,the polarization-dependent di ff erential cross section is quite sensitive to the reagent vibrational excitation.Based on the obtained results,we fi nd that the observed excess of the methylidyne cation CH+is closely related to the reactant of vibrational excitation in interstellar chemistry.

molecular reaction dynamics,quasi-classical trajectory method,vector correlation

10.7498/aps.66.123401

?國(guó)家自然科學(xué)基金(批準(zhǔn)號(hào):21403226,21503226)資助的課題.

?通信作者.E-mail:huzi233@126.com

?2017中國(guó)物理學(xué)會(huì)Chinese Physical Society

http://wulixb.iphy.ac.cn

*Project supported by the National Natural Science Foundation of China(Grant Nos.21403226,21503226).

?Corresponding author.E-mail:huzi233@126.com