Transcript VUV-MATI-PD for Ion Reaction Control

VUV-MATI-PD for Ion Reaction Control
National Creative Research Initiative for Control of Reaction Dynamics
and School of Chemistry, Seoul National University, Seoul 151-742, 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- C3H7I+•.
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
5
4
3
2
1
=0
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
A+BC
2
1
RBC
ABC
RAB
 Pumping 4 vibrational quanta to (OH)  Y(OD)/Y(OH) ~ 15
 Limited to small system
 IVR is still a problem
2. Active Control
Utilize special properties of laser to manipulate the motion
of electrons and nuclei
f

Brumer-Shapiro scheme
Exploits interference between two independent
pathways that connect the same initial and final
states. 3+3 1 most popular

1
1
3
1
i
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
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
High resolution spectrum of M+•
2. Rydberg states
H-like atom states with one e- in a high n orbital
Enlm 
eM+•
 RM
(n   l ) 2
 r  nlm  n 2 {1 
1
l (l  1)
(1 
)}a0
2
n2
RM : Mass-dependent Rydberg constant
M+• : Ionic core
( R∞ = 109737 cm-1)
e- : Rydberg electron
 : -dependent quantum defect
a0 : Bohr radius (0.529 Å)
States
Enlm
<r>nlm
n=200, =0
-3 cm-1
2 m
n=2000, =0
-0.003 cm-1
2 mm
Rydberg Series

Series of Rydberg states with different n converging to an
ionization limit. Each ionic state has a corresponding
Rydberg series
2
1
Ionic state 0
h

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)
Lifetime of a Rydberg state
 ∝n3
From extrapolation of experimental data of NO
n=200, p-orbital
 = 100 nsec
n=2000, p-orbital
 = 100 sec
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
3. Field-ionization

Pulsed-field ionization(PFI) after a time delay
U
Potential energy of Rydberg electron
E
in the presence of applied field F
e2
U 
 eFr
4r
E(cm-1) ∼4 F , F in Vcm-1
F ,Vcm-1
0.01
0.1
1
100
E,cm-1
0.4
1.3
4
40
n
500
300
170
50

Ionization by stray electric field
Even with best effort, stray field present in the apparatus > 20 mVcm-1
→ E = 0.6 cm-1
→ 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.
Must be removed before PFI.
Technique
Delay PFI until direct electrons are removed by e--e- repulsion or
by stray field.
R ee-R e-
ee- R
e- R
R
R
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?
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  ↑, <r> ↑).
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
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 eGeneration 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
Since low n Rydbergs are depleted by intramolecular relaxation,
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
Photoexcitation
E1
E1
950V
E2
E2
h
M
E3
E3
1200V
-1V
PFI delay
Simultaneous pulsing of E2 and E3 voltages needed for time
focusing of TOF peaks.
→ Technically difficult to apply additional AC field.

Use of electronic jitter
E1
photoexcitation
950V
E2
1200V
E3
PFI delay
Switch off E2 voltage just before photoexcitation.
→ Weak voltage ringing, jitter, serves as scrambling field.
number of ions
30000
NO+ - regular MATI
(a)
20000
10000
0
-10
0
10
20
30
40
50
PFI delay (s)
number of ions
30000
NO+ - MATI with scrambling field
(b)
20000
10000
0
-10
0
10
20
30
40
50
PFI delay (s)
number of ions
15000
NO+ - direct ions
(c)
10000
5000
0
-10
0
10
20
PFI delay (s)
30
40
50
III. Generation of coherent vacuum ultraviolat (VUV)
radiation by four-wave mixing
A. Popular photoexcitation scheme for ZEKE/MATI
h2
Two-photon ‘1+1’
IE
h1
Difficulties
1. Difficult to control multiphoton processes.
2. Applicable to systems with a stable intermediate state
with E < 220 nm = 5.6 eV.
eg. C6H6, IE=9.23 eV, S1-S0 = 4.72 eV = 263 nm
IE –S1 = 4.51 eV = 275 nm
Most system, S1 – S0 < 200 nm, diffuse S1.

Two-photon ‘2+1’ is even worse
B. One-photon ZEKE/MATI with VUV
Typical IE = 9~12 eV = 138 ~ 103 nm
1. VUV generation by four-wave difference frequency mixing in Kr
5p[1/2]
0
5p[5/2]
h3
2
h2
h1 = h2 = 212.6nm or 216.7 nm
h4
h1
4p6
h3 = 400~800 nm
h4 = 122 ~145 nm, 10 nJ
2. VUV generation by sum frequency mixing in Hg
h3
71S0
h1 = h2 = 312.8 nm
h2
h4 = 115~125nm
h4
h1
MCP
TOF
61S0
Ar
Out
Out
LiF lens
UV, S
Achromatic
lens
Water in
Hg
Heating
block
Water in
Temperature-controlled
pulsed valve
IV. One-photon VUV-MATI spectroscopy
A. Experimental
(a) top view
(b) side view
detector
photoionization
chamber
MgF2
lens
Kr cell
dichroic mirror
50cm lens
TOF
molecular beam
G
E1
E2
E3
VUV
B. VUV-MATI spectroscopy of 2-iodopropane
Two ionization thresholds for the ground spin-orbit doublet of 2-C3H7I+•.
IE(X1) ~ 9.2 eV = 135 nm = 74000 cm-1
IE(X2) ~ 9.7 eV = 128 nm = 78100 cm-1
Photon energy

C3H7+ : Dissociation of ionic core of Rydberg neutral
e-
eC3H7I+

C3H7+ + I
Dissociation threshold determined
PFI
C3H7+ + I
C2H5I
IE (X1)a
9.3490  0.0005
1-C3H7I
9.2567  0.0005 (G)
9.2718  0.0005 (T)
2-C3H7I
9.1755  0.0005
9.3492  0.0006
9.35  0.01
IE (X2)a
9.9327  0.0017
9.25  0.01
9.26  0.01
9.8332  0.0017 (G)
9.8466  0.0017 (T)
9.19  0.01
9.18  0.01
9.6903  0.0017
Ref
this work
this work
19
9
20
9.84  0.01
9.82  0.01
9.77  0.02
9.75  0.01
this work
this work
19
9
20
9.8332  0.0017
9.8180  0.0037
9.851  0.025
9.77  0.02
9.82  0.01b
this work
11
9
8
9.9324  0.0006
9.93  0.01
AE(C3H7+)
9.84  0.01
C. VUV-MATI spectroscopy of 1-iodopropane
IE(X1) ~ 9.25 eV = 134 nm = 74600 cm- 1
IE(X2) ~ 9.84 eV = 126 nm = 79400 cm-1
Two major peaks in Fig 4 (a)
gauche, 74660 cm-1
anti, 74790 cm-1
D. VUV-MATI spectroscopy of iodobutane
1. iso-Butyl iodide
73972 cm-1
74171 cm-1
2. 2-Iodobutane
 Conformation assignment not possible
3. 1-Iodobutane
 Conformation assignment not possible
V. Photodissociation of conformation-selected 1- C3H7I+•.
A. Introduction

Gauche and anti ions without any internal energy are formed
→ No interconversion between conformers

Ions prepared under very high vacuum condition
→ No collision-induced interconversion

For a dissociation occurring faster than interconversion,
conformation-specificity may be observed
→ Excitation to a repulsive electronic state
B. Experimental
detector
photoionization
chamber
MgF2
lens
Kr cell
dichroic mirror
50cm lens
photodissociation laser
TOF
molecular beam
G
E1
E2
E3
VUV
photodissociation laser
(a)
(a)(a)
Gauche
Gauche
Gauche
ion signal, a.u.
607nm
1- C3H7I+•
(a)
(b)
9.40
(b)

9.40
Gauche
Anti
0◦
35◦
55◦
90◦
9.42
9.44
time of flight,s
9.46
9.48
ion signal, a.u.
ion signal, a.u.
ion signal, a.u.
C. Photodissociation TOF profiles
C3 H7 + + I
•
Anti
Anti
(b)(b)
0◦
90◦
9.40
9.40
9.42
9.42
Anti
9.44
9.44
9.46
9.46
TOF profiles of C3H7+ broadened due to kinetic energy release (KER,T)
TOF profiles also affected by polarization of PD laser
→ anisotropic dissociation ( reaction time < rotational period )
9.42
9.44
9.46
9.48
time of flight,s

9.48
9.48
time
flight,
time
of of
flight,
ss
T &  (anisotropy parameter) for gauche > anti
Reaction time < time for interconversion between conformers
D. Distributions of T & 
30
(a)
probability
1.0
20
0.8
10
0.6
0
anisotropy
(b)
Gauche
Anti
0.4
0.2
0.0
0.0
0.1
0.2
0.3
kinetic energy release, eV
Distributions of (a) T and (b)  obtained by analyzing the TOF profiles of C3H7+ at
607 nm in Fig. 2. The results for the gauche and anti conformations are shown as
the open and filled circles, respectively.
(a)
0.15
0.10
0.4
0.05
0.2
0.00
0.0
1.0
(b)
0.6
anisotropy
kinetic energy release, eV
E. <T> & < > vs internal energy
1.5
2.0
2.5
photon energy, eV
3.0
1.0
1.5
2.0
2.5
photon energy, eV
(a) <T> and (b) <> vs. photon energy (480 ~ 700 nm) for the photodissociation of
C3H7I+ to C3H7+ + I. The results for the gauche and anti conformations are shown
as the open and filled circles, respectively. Some <T> data from the
photoelectron-photoion coincidence spectrometric measurement by Brand and
coworkers in ref. 7 are shown as open triangles (∆) in (a).

Dissociation threshold
Gauche ~ 1.3 eV
Anti ~ 1.6 eV
3.0
F. Thermochemistry
h

1-C3H7I+•
1- C3H7+ + I• ?
1-C3H7+ is not a stable species!
2-C3H7+ and cyclo- C3H7+ ( protonated cyclopropane) are stable

Four possibilities
(1) 2-C3H7+
(2) cyclo-C3H7+
(3) 2-C3H7+
(4) cyclo-C3H7+

+
+
+
+
I
I
I
I
(2P3/2)
(2P3/2)
(2P1/2)
(2P1/2)
Excited state where dissociation occurs is the same for gauche
and anti. → Iodine state same.
∴ Either (1) & (2) or (3) & (4)
System
H 0f ,0K (eV)†
0
H 0K
, gauche (eV)
0
H 0K
, anti (eV)
Reactant
1-C3H7I+(gauche)
9.170  0.040
1-C3H7I+ (anti)
9.186  0.040
Product
2-C3H7+ + I (2P3/2)
9.627  0.039
0.457  0.056
0.442  0.056
c-C3H7+ + I (2P3/2)
9.958  0.041
0.788  0.056
0.772  0.056
2-C3H7+ + I (2P1/2)
10.570  0.039
1.400  0.056
1.385  0.056
c-C3H7+ + I (2P1/2)
10.901  0.041
1.730  0.056
1.715  0.056
†
Enthalpy of formation at 0 K. For products, it is the sum of the two. Data for the reactants and 2-C3H7+
(8.517 eV) are evaluated with thermochemical data in ref. 6 and ref. 22. Enthalpy of formation at 0 K of cC3H7+, 8.847 eV, is evaluated using the ab initio results at the G2 level. The energy difference between the two
fragment ions, 0.331 eV, is close to the ab initio result, 0.313 eV, at the MP4/6-311G** level, ref. 16. Using the
enthalpy of formation of c-C3H7+ obtained from the proton affinity measurement in ref. 23, 8.864 eV, the
experimental difference becomes 0.347 eV, which is in decent agreement with the G2 result. Enthalpies of
formation at 0 K of I (2P3/2) and I (2P1/2) are 1.1107 and 2.0534 eV, respectively, ref. 24.
Best candidates
gauche → 2- C3H7+ + I (2P1/2)
anti → cyclo- C3H7+ +I (2P1/2)
G. Ab initio calculation
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-C3H7I+. 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-C3H7+ + I, is taken as the zero of the energy scale. Some
representative geometries are also drawn.

Intramolecular SN2-type rearrangement accompanies the
C-I bond breaking, both for gauche and anti.
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.
Participants
Prof. Hong Lae Kim , Kangwon National Univ.
Prof. Sang Kyu Kim , Inha Univ.
Dr. Wan Goo Hwang
Dr. Sang Tae Park
Chan Ho Kwon