VUV-MATI-PD for Ion Reaction Control National Creative Research Initiative for Control of Reaction Dynamics and School of Chemistry, Seoul National University, Seoul , Korea Prof. Myung Soo Kim

Contents I.Reaction control II.Mass-analyzed threshold ionization (MATI) for generation of state-selected ion beam III.Generation of coherent vacuum ultraviolet (VUV) radiation by four-wave mixing IV.One-photon VUV-MATI spectroscopy V.Photodissociation of conformation-selected 1- C 3 H 7 I +. VI.Summary VII.Acknowledgments

I. Reaction Control A. Perspective  One main goal of chemistry – Efficient production of any useful material as one wishes  Traditional approach – Change T, P, Catalyst, etc.  Dynamic approach – Utilize dynamic character of a reaction. For example, change the initial quantum state of the reactant system or utilize the special properties of laser which interacts with the system

B. Earlier(1970~1990) attempts on laser control of chemical reactions Mode Selective Chemistry Can we selectively break Y-H bond of X-Y-H by pumping many IR photons and exciting Y-H stretching vibration, (YH)? One of the difficulties – Multiphoton excitation with one IR laser to highly excited (YH) state is not possible due to anharmonicity An alternative – Overtone excitation ( eg,  =0 →  =10) using tunable dye laser in VIS  =     

IVR ( Intramolecular Vibrational Redistribution ) – Main difficulty Vibrational energy supplied to a particular mode by selective pumping is redistributed rapidly to other modes within several psec. More or less statistical distribution of internal energy RRKM-type(statistical) reaction rather than mode-selective reaction

C. Recent approach – passive and active controls 1.Passive control – state-selective chemistry Prepare the system close to the path of a particular reaction Vibrational state control of HOD photodissociation – F. Flemming Crim Vibrational state control of HOD photodissociation – F. Flemming Crim  Pumping 4 vibrational quanta to (OH)  Y(OD)/Y(OH) ~ 15  Limited to small system  IVR is still a problem A+BC ABC 1 2 R AB R BC

Utilize special properties of laser to manipulate the motion of electrons and nuclei  Brumer-Shapiro scheme Exploits interference between two independent pathways that connect the same initial and final states.  3 +3  1 most popular  Rice-Tannor-Kosloff-Rabitz scheme Pump and dump by femtosecond laser pulses for reaction control Optimization of laser pulse shape is thought to be critical 2. Active Control i f 33 11 11 11

II. Mass-analyzed threshold ionization (MATI) for generation of state-selected ion beam A. Zero electron kinetic energy (ZEKE) spectroscopy 1. Outline Step1 – Excite a neutral (M) to a very high Rydberg state Step2 – Ionize it with electric field ( field-ionization) Step3 – Detect e - signal vs. h Step3 – Detect e - signal vs. h High resolution spectrum of M + High resolution spectrum of M +

2. Rydberg states H-like atom states with one e - in a high n orbital M+ e-e- M+ : Ionic core e - : Rydberg electron R M : Mass-dependent Rydberg constant ( R ∞ = cm -1 )  : -dependent quantum defect a 0 : Bohr radius (0.529 Å )States E nlm nlm nlm n=200, =0 -3 cm -1 2  m n=2000, = cm -1 2 mm

 Series of Rydberg states with different n converging to an ionization limit. Each ionic state has a corresponding Rydberg series  Continuum of states ( ionization continuum) is present above each ionization threshold.  Irradiation with h shown above Generation of a Rydberg neutral converging to ionic state 2 and M + in state 0 or 1 (direct photoionization) Generation of a Rydberg neutral converging to ionic state 2 and M + in state 0 or 1 (direct photoionization) Rydberg Series Ionic state h

Lifetime of a Rydberg state  ∝ n 3 From extrapolation of experimental data of NO If we wait for a long time after excitation, only those Rydberg states very close to the ionization threshold will survive. Then, field-ionization  High resolution spectrum n=200, p-orbital  = 100 nsec n=2000, p-orbital  = 100  sec

3. Field-ionization  Pulsed-field ionization(PFI) after a time delay Potential energy of Rydberg electron in the presence of applied field F EE U  E(cm -1 ) ∼ 4, F in Vcm -1 F,Vcm  E,cm n

 Ionization by stray electric field Even with best effort, stray field present in the apparatus > 20 mVcm -1 →  E = 0.6 cm -1 →  E = 0.6 cm -1 → Rydberg states with n > 400 undergo ionization by stray field. → Rydberg states with n > 400 undergo ionization by stray field. It is thought that ZEKE detects Rydberg with n= 200~300 ( 1~3 cm -1 below the ionization thresold) with  = 100~300 nsec (?).

4. Removal of direct electrons Direct electrons : electrons formed by direct PI. Always generated together with Rydbergs. Always generated together with Rydbergs. Must be removed before PFI. Must be removed before PFI. Technique Delay PFI until direct electrons are removed by e - -e - repulsion or by stray field. How much delay? n=200~300 →  =100~200 nsec But, longer delay time, 1~10  sec, used for ZEKE. Namely, Rydberg survive much longer than theoretically expected. Why? R R e-e- e-e- e-e- e-e- e-e- e-e- R R R R

5. -mixing by Stark effect and ZEKE states Rydberg states prepared by photoexcitation → large n, small ( ∵ ∆ =  1) Stark effect by stray field → -mixing → Relaxation to high states. Inhomogeneous field by charged particles → m-mixing ZEKE states High n,, m states Weak interaction between ionic core and Rydberg e - ( ∵ As ↑, ↑). Slow autoionization, internal conversion, etc. Lifetime lengthened by ~ n. Eg) n=200 Rydberg state with =1 →  ~ 100 nsec n=200 ZEKE state with high →  ~ 20  sec n=200 ZEKE state with high →  ~ 20  sec

6. ZEKE spectroscopy  PFI with 1~10  sec delay after photoexcitation  Field-ionization of ZEKE states with n=200~300  Low voltage electronics for PFI and e - acceleration

B. Mass-analyzed threshold ionization (MATI) 1. Principle Same as ZEKE. Detects M + generated by PFI, not e - Generation of state-selected M + ion beam 2. Difficulty  Direct ion, M + generated by direct PI, must be removed. Difficult because mass(M + ) ≫ mass(e - )  Technique Apply a weak DC field (spoil field,~1Vcm -1 ) for 1~10  sec to remove M +. → Depletion of high n Rydbergs by field-ionization Apply a weak DC field (spoil field,~1Vcm -1 ) for 1~10  sec to remove M +. → Depletion of high n Rydbergs by field-ionization Since low n Rydbergs are depleted by intramolecular relaxation, Since low n Rydbergs are depleted by intramolecular relaxation, not many Rydbergs are left for PFI. not many Rydbergs are left for PFI.  Poor signal intensity!

3.,m-mixing by AC field  Application of a weak AC field (scrambling field) further mixes m states → Further lengthening of lifetime.  Regular MATI scheme Simultaneous pulsing of E2 and E3 voltages needed for time focusing of TOF peaks. Simultaneous pulsing of E2 and E3 voltages needed for time focusing of TOF peaks. → Technically difficult to apply additional AC field. → Technically difficult to apply additional AC field. E1 E2 E3 1200V 950V -1V PFI delay E3 E2 E1 h M Photoexcitation

 Use of electronic jitter Switch off E2 voltage just before photoexcitation. → Weak voltage ringing, jitter, serves as scrambling field. E1 E2 E3 1200V 950V photoexcitation PFI delay

III. Generation of coherent vacuum ultraviolat (VUV) radiation by four-wave mixing A. Popular photoexcitation scheme for ZEKE/MATI Two-photon ‘ 1+1 ’ Two-photon ‘ 1+1 ’Difficulties 1. Difficult to control multiphoton processes. 2. Applicable to systems with a stable intermediate state with E < 220 nm = 5.6 eV. with E < 220 nm = 5.6 eV. eg. C 6 H 6, IE=9.23 eV, S 1 -S 0 = 4.72 eV = 263 nm eg. C 6 H 6, IE=9.23 eV, S 1 -S 0 = 4.72 eV = 263 nm IE – S 1 = 4.51 eV = 275 nm IE – S 1 = 4.51 eV = 275 nm Most system, S 1 – S 0 < 200 nm, diffuse S 1. Most system, S 1 – S 0 < 200 nm, diffuse S 1.  Two-photon ‘ 2+1 ’ is even worse h 1 h 2 IE

B. One-photon ZEKE/MATI with VUV Typical IE = 9~12 eV = 138 ~ 103 nm Typical IE = 9~12 eV = 138 ~ 103 nm 1. VUV generation by four-wave difference frequency mixing in Kr h 1 h 2 h 3 h 4 4p 6 5p[5/2] 2 5p[1/2] 0 h 1 = h 2 = 212.6nm or nm h 3 = 400~800 nm h 4 = 122 ~145 nm, 10 nJ

2. VUV generation by sum frequency mixing in Hg h 1 h 2 h 3 h 4 61S061S0 71S071S0 h 1 = h 2 = nm h 4 = 115~125nm

IV. One-photon VUV-MATI spectroscopy A. Experimental (a) top view dichroic mirror Kr cell MgF 2 lens photoionization chamber 50cm lens (b) side view detector molecular beam VUV E3 E2 E1 G TOF

B. VUV-MATI spectroscopy of 2-iodopropane Two ionization thresholds for the ground spin-orbit doublet of 2-C 3 H 7 I +. IE(X 1 ) ~ 9.2 eV = 135 nm = cm -1 IE(X 1 ) ~ 9.2 eV = 135 nm = cm -1 IE(X 2 ) ~ 9.7 eV = 128 nm = cm -1 IE(X 2 ) ~ 9.7 eV = 128 nm = cm -1  C 3 H 7 + : Dissociation of ionic core of Rydberg neutral  Dissociation threshold determined e-e- e-e- C3H7I+C3H7I+ C 3 H I PFI Photon energy

C2H5IC2H5IC2H5IC2H5I 1-C 3 H 7 I 2-C 3 H 7 I Ref IE (X 1 ) a        (G)  (G)  (T)  (T) 9.25       0.01 this work this work this work IE (X 2 ) a        (G)  (G)  (T)  (T) 9.84      0.01 this work this work this work AE(C 3 H 7 + )        0.01 b 9.82  0.01 b this work 1198

C. VUV-MATI spectroscopy of 1-iodopropane IE(X 1 ) ~ 9.25 eV = 134 nm = cm - 1 IE(X 2 ) ~ 9.84 eV = 126 nm = cm -1 Two major peaks in Fig 4 (a) gauche, cm -1 anti, cm -1

D. VUV-MATI spectroscopy of iodobutane 1. iso-Butyl iodide cm cm -1

2. 2-Iodobutane  Conformation assignment not possible

3. 1-Iodobutane  Conformation assignment not possible

V. Photodissociation of conformation-selected 1- C 3 H 7 I +. A. Introduction  Gauche and anti ions without any internal energy are formed → No interconversion between conformers → No interconversion between conformers  Ions prepared under very high vacuum condition → No collision-induced interconversion → No collision-induced interconversion  For a dissociation occurring faster than interconversion, conformation-specificity may be observed → Excitation to a repulsive electronic state → Excitation to a repulsive electronic state

B. Experimental dichroic mirror Kr cell MgF 2 lens photoionization chamber 50cm lens photodissociation laser detector molecular beam VUV E3 E2 E1 G TOF photodissociation laser

C. Photodissociation TOF profiles 1- C 3 H 7 I + C 3 H I 1- C 3 H 7 I + C 3 H I  TOF profiles of C 3 H 7 + broadened due to kinetic energy release (KER,T) TOF profiles also affected by polarization of PD laser → anisotropic dissociation ( reaction time < rotational period ) → anisotropic dissociation ( reaction time < rotational period )  T &  (anisotropy parameter) for gauche > anti Reaction time < time for interconversion between conformers 607nm 90 ◦ 55 ◦ 35 ◦ 0◦0◦ 90 ◦ 0◦0◦

D. Distributions of T &  Distributions of (a) T and (b)  obtained by analyzing the TOF profiles of C 3 H 7 + at 607 nm in Fig. 2. The results for the gauche and anti conformations are shown as the open and filled circles, respectively.

E. & vs internal energy  Dissociation threshold Gauche ~ 1.3 eV Gauche ~ 1.3 eV Anti ~ 1.6 eV Anti ~ 1.6 eV (a) and (b) vs. photon energy (480 ~ 700 nm) for the photodissociation of C 3 H 7 I +  to C 3 H I . The results for the gauche and anti conformations are shown as the open and filled circles, respectively. Some data from the photoelectron-photoion coincidence spectrometric measurement by Brand and coworkers in ref. 7 are shown as open triangles (∆) in (a).

F. Thermochemistry  1-C 3 H 7 I + 1- C 3 H I ? 1-C 3 H 7 + is not a stable species! 1-C 3 H 7 + is not a stable species! 2-C 3 H 7 + and cyclo- C 3 H 7 + ( protonated cyclopropane) are stable 2-C 3 H 7 + and cyclo- C 3 H 7 + ( protonated cyclopropane) are stable  Four possibilities (1) 2-C 3 H I ( 2 P 3/2 ) (1) 2-C 3 H I ( 2 P 3/2 ) (2) cyclo-C 3 H I ( 2 P 3/2 ) (2) cyclo-C 3 H I ( 2 P 3/2 ) (3) 2-C 3 H I ( 2 P 1/2 ) (3) 2-C 3 H I ( 2 P 1/2 ) (4) cyclo-C 3 H I ( 2 P 1/2 ) (4) cyclo-C 3 H I ( 2 P 1/2 )  Excited state where dissociation occurs is the same for gauche and anti. → Iodine state same. ∴ Either (1) & (2) or (3) & (4) ∴ Either (1) & (2) or (3) & (4) h

Best candidates gauche → 2- C 3 H I ( 2 P 1/2 ) gauche → 2- C 3 H I ( 2 P 1/2 ) anti → cyclo- C 3 H 7 + +I ( 2 P 1/2 ) anti → cyclo- C 3 H 7 + +I ( 2 P 1/2 )System (eV) † (eV) †, gauche (eV), gauche (eV), anti (eV), anti (eV) Reactant 1-C 3 H 7 I +  (gauche)   C 3 H 7 I +  (anti)   Product 2-C 3 H I ( 2 P 3/2 )      c-C 3 H I ( 2 P 3/2 )      C 3 H I ( 2 P 1/2 )     c-C 3 H I ( 2 P 1/2 )     † Enthalpy of formation at 0 K. For products, it is the sum of the two. Data for the reactants and 2-C 3 H 7 + (8.517 eV) are evaluated with thermochemical data in ref. 6 and ref. 22. Enthalpy of formation at 0 K of c-C 3 H 7 +, eV, is evaluated using the ab initio results at the G2 level. The energy difference between the two fragment ions, eV, is close to the ab initio result, eV, at the MP4/6-311G** level, ref. 16. Using the enthalpy of formation of c-C 3 H 7 + obtained from the proton affinity measurement in ref. 23, eV, the experimental difference becomes eV, which is in decent agreement with the G2 result. Enthalpies of formation at 0 K of I ( 2 P 3/2 ) and I ( 2 P 1/2 ) are and eV, respectively, ref. 24.

G. Ab initio calculation  Intramolecular S N 2-type rearrangement accompanies the C-I bond breaking, both for gauche and anti. C-I bond breaking, both for gauche and anti. Potential energy along the reaction path. Changes in potential energies along the minimum energy paths for the dissociations of gauche and anti isomers in the first excited state were obtained by ab initio calculation at the CIS level. The 6-31G ** basis set was used for carbons and hydrogens, while the LanL2DZ basis set was used for iodine. Equilibrium geometries of the gauche and anti isomers in the ground electronic state at the Hartree-Fock level were taken as the initial geometries of the photoexcited 1-C 3 H 7 I + . Then, the energies and gradients in the first excited state corresponding to the above configurations were calculated by CIS. Finally, the minimum energy paths from these configurations were calculated by the steepest descent method. The energy of the products, 2- C 3 H I , is taken as the zero of the energy scale. Some representative geometries are also drawn.

VI. Summary A. VUV-MATI useful to obtain accurate ionization energies to the ground and some excited electronic states, vibrational frequencies, dissociation thresholds. B. Conformation-selected ion beam generated for haloalkane ions C. Conformation-specific reaction observed for the first time. It has been clearly demonstrated that conformation can be gateways to different reactions as has been long postulated in stereochemistry D. Conformation-specificity, a well-known concept in chemistry, can be a useful alternative to more elaborate control schemes presented so far.

VII. Acknowledgments This work was supported financially by CRI, the ministry of Science and Technology, Republic of Korea. This work was supported financially by CRI, the ministry of Science and Technology, Republic of Korea. Participants Participants Prof. Hong Lae Kim, Kangwon National Univ. Prof. Hong Lae Kim, Kangwon National Univ. Prof. Sang Kyu Kim, Inha Univ. Prof. Sang Kyu Kim, Inha Univ. Dr. Wan Goo Hwang Dr. Wan Goo Hwang Dr. Sang Tae Park Dr. Sang Tae Park Chan Ho Kwon Chan Ho Kwon