Probing very long-lived excited electronic states of molecular cations by mass spectrometry School of chemistry and National Creative Research Initiative for Control of Reaction Dynamics, Seoul National University, Seoul , Korea Prof. Myung Soo Kim

I. Introduction  Involved in various processes such as photochemistry, operation of lasers, etc.  Difficult to probe. Information scarce.  A frontier in physical chemistry research For example, accurate and efficient calculation of excited state energy is the main focus in quantum chemistry.  Our interest  Utilization of excited electronic states for reaction control A. Excited electronic states

B. Fate of an isolated polyatomic system prepared in an excited electronic state 1.Nonradiative decay Internal conversion / intersystem crossing convert the electronic energy into vibrational energy in the ground electronic state. 2. Direct photodissociation on a repulsive state Utilized in our previous work on reaction control via conformation selection (Nature 415, 306 (2002)). 3. Radiative decay – fluorescence Occurs when nonradiative decay is not efficient and electric dipole – allowed transition is present.

C. Excited electronic states of molecular ions  Electron ionization (EI) and VUV photoionization (PI) generate hole states mostly. Peaks in photoelectron spectrum  hole states.  There are more excited electronic states near the ground state of a molecular ion than that of a neutral ( presence of hole states).  Rapid internal conversion prevalent. Fluorescence hardly observed for polyatomic molecular cations.  LUMO HOMO Hole statesLUMO states

D. Theory of mass spectra 1)Molecular ions in various electronic (and vibrational) states are produced by EI (or PI). 2)Ions in excited electronic states undergo rapid internal conversion to the ground state.  Rapid conversion of electronic energy to vibrational energy. 3)Intramolecular vibrational redistribution (IVR) occurs rapidly also.  Transition state theory, or, Rice-Ramsperger-Kassel-Marcus (RRKM) theory.  QET or RRKM – QET 1. Quasi-equilibrium theory (QET)

 Prepare M + with different E.  Measure or product branching ratios vs. E.  Compare with the calculated results. 2. Test 3. Results  RRKM-QET adequate for most of the cases studied.  Some exceptions observed. : Mostly direct dissociation in repulsive excited states. In several cases, dissociation in excited states which do not undergo rapid internal conversion to the ground state suggested. ‘Isolated electronic state’

II. Initial discovery A. Photodissociation of benzene cation  Observed C 6 H 6 +  C 6 H 5 +, C 6 H 4 +, C 4 H 3 +, C 3 H 3 + at 514.5nm (2.41eV), 488.0nm (2.54eV), 357nm (3.47eV)  Instrument can detect PD occurring within ~1  sec. Magnetic sector Ion source Electrode assembly Electric sector PMT Chopper Lens Argon ion laser Phase- sensitive detection Laser beam Prism Laser beam Collision cell R1 R2 R3R6R4R7R5 Ion beam Ion beam Schematic diagram of the double focusing mass spectrometer with reversed geometry (VG ZAB-E) modified for photodissociation study. The inset shows the details of the electrode assembly. J. Chem. Phys. 113, 9532 (2000)

 For PD to be observed with the present apparatus, photoexcited C 6 H 6 + must have E > 5 eV Remainder ?  Photon energy = 2.4 ~ 3.7 eV. Energy diagram of the benzene molecular ion. The lowest reaction threshold (E 0 ) is 3.66 eV for C 6 H 6   C 6 H 5   H . k tot denotes the total dissociation rate constant in the ground state calculated from previous results. X 2 E 1g (ground state) B 2 E 2g C 2 A 2u D 2 E 1u E(eV) 0 2 Dissociation ( Products ) ~ ~ ~ ~ E 2 B 2u ~ Electronic states ( C 6 H 6 + ) k tot ~ 10 7 s -1 k tot ~ 10 4 s -1 C 6 H 6 +  C 6 H H

PD-MIKE profile for the production of C 4 H 4  from the benzene ion at 357nm obtained with 2.1kV applied on the electrode assembly. Experimental result is shown as filled circles. Reproduction of the profile using the rate constant distribution centered at 6.3  10 7 s -1 obtained by experimental data is shown as the solid curve. The positions marked A and B are the kinetic energies of products generated at the position of photoexcitation and after exiting the ground electrode, respectively.

 Excellent RRKM – QET fitting of is known for C 6 H 6 + dissociation.  From measured  E PD at 357nm (3.47eV)  E=6.1 ± 0.1eV  Initial E = 2.6 ± 0.1eV PD at 488nm (2.54eV)  E=5.5 ± 0.1eV  Initial E = 3.0 ± 0.1eV The total RRKM dissociation rate constant of BZ  as a function of the internal energy calculated with molecular parameters in ref. 8. The internal energies corresponding to the dissociation rate constants of (5.5  1.1)  10 7 and (5  3)  10 6 s -1 for PDs at 357 and nm, respectively, are marked.

Origin of internal E prior to photoexcitation Most likely  vibrational energy acquired at the time of EI, either directly or via internal conversion from an excited electronic state. 2.6  0.1 eV for 357nm PD vs. 3.0  0.1 eV for 488nm PD ? Experimental error? Can we quench it by increasing benzene pressure in the ion source, by resonant charge exchange ? C 6 H 6 + * + C 6 H 6  C 6 H 6 * + C 6 H 6 +

PD as a function of C 6 H 6 pressure in the ion source Pressure dependences of the precursor (BZ  ) intensity (–––) and photoproduct (C 4 H 4  ) intensities at 357 (·····) and (---) nm. Pressure in the CI source was varied continuously to obtain these data. Pressure was read by an ionization gauge located below the source. The inside source pressures estimated at three ionization gauge readings are marked. The scale for the precursor intensity is different from that for photoproduct intensities. Ion source pressure (P), collision frequency (Z c ), source residence time (t R ), and number of collisions (N coll ) suffered by ions exiting the ion source at some benzene pressures. P ig /TorrP/Torr Z c /  s -1 tR/stR/s N coll 4      

Quenching mechanism  PD at 488nm efficiently quenched (by every collision)  resonant charge exchange likely.  PD at 357nm hardly quenched. Why? If C 6 H 6 + undergoing PD at 357nm is in an excited electronic state, C 6 H 6 +† + C 6 H 6  C 6 H 6 + C 6 H 6 +† Population of C 6 H 6 +† does not decrease by charge exchange.

Charge exchange ionization by benzene cation in the ion source  One of the ionization scheme classified as chemical ionization (CI), a useful ionization technique in mass spectrometry.  Add small amount of sample (s) to reagent (R)  Electron ionization  Initially, R + formed mostly.  Charge exchange ionization of S by R + R + + S  R + S +, electron transfer Translational & vibrational energies are not important to drive this reaction Occurs efficiently when, exoergic reactions.

Relative intensity of S + formed by charge exchange with C 6 H 6 +  At low C 6 H 6 pressure in the source  PD at 357nm occurs  Possible presence of long-lived C 6 H 6 +, C 6 H 6 +†.  At high C 6 H 6 pressure  complete quenching of PD at 357nm  absence of C 6 H 6 +†. Ionization Energies and the ratios of molecular ion intensities generated by charge exchange ionization (CI) with BZ  and by electron ionization (EI).

 C 6 H 6 + generated at high P, fully quenched  ionizes samples with IE < 9.2eV. cf. IE (C 6 H 6 ) = 9.243eV  C 6 H 6 + generated at low P  ionizes samples with IE < 11.5 eV. cf. IE of C 6 H 6 to state of C 6 H 6 = eV

B. Summary Low-lying excited electronic states of C 6 H 6 +  has a very long lifetime, ‘isolated state’.   electric dipole – forbidden. Internal conversion must be inefficient also.  For states above,internal conversion efficient. (Evidence – failure to ionize S with IE > 11.5 eV by charge exchange) IE = eV IE = eV IE = 12.3 eV

Sharp vibrational peaks for and. C 6 H 6 Photoelectron Spectrum

III. Charge exchange ionization to detect M +† 1. Energetics A + + B  A + B +,  E, energy defect For A + in the ground state,  E > 0, endoergic = 0, resonant < 0, exoergic J.Am. Soc. Mass Spectrom. 12, 1120 (2001).

2. Charge exchange cross section 1)Charge exchange between atomic species Massey’s adiabatic maximum rule Maximum cross section (  max ) occurs at the velocity For ~ 0,  max observed v ~ 0 Otherwise,  max observed at high v

2) Charge exchange involving molecular species  Release of as product vibration  Energetically nearly resonant  large  at near thermal velocity Endoergic charge exchange (  E > 0)  Small  at near thermal velocity. Usually keV impact energy needed.  Reactant vibrational energy sometimes helps to increase , but not dramatically. Exoergicity rule For near thermal collision  large when  E  0  small when  E > 0 Exoergic charge exchange (  E < 0)

3. Instrumentation Collision cell for conventional tandem mass spectrometry 1) Requirement G Charge exchange  For charge exchange at low impact energy, M + must be decelerated.  Should detect G +, which moves thermally inside the cell.  Low yield.

2) Instrumentation First collision cell Ion beam Ion source Magnetic sector Conversion dynode EM Electric sector Repeller Ion Source First collision cell Conversion dynode PM Second collision cell Collision Cell Y-lens

3) First Cell Type I ions ( formed by EI in the source ) K I = eV s Type II ions ( formed by CID in the cell ) K II = e [V s +(m 1 /M)(V s -V c )] Type III ions ( formed from collision gas ) K III = eV c Magnetic analyzer : m/z = B 2 r 2 e 2 /2K VsVs M+M+ Magnetic analyzer VcVc Ion source Collision cell

4) Second Cell VsVs VcVc Ion source  Magnetic analyzer Electrostatic analyzer Collision cell  Select by magnetic analyzer.  Measure ion kinetic energy by electrostatic analyzer.  Detect ions generated from collision gas ( KE of type III differs from those of Type I & II) 

4. Charge exchange data for C 6 H 6 +  1) Second cell RE (C 6 H 6 +, ) = eV IE (CS 2 ) =10.07 eV  E = = eV Exoergic ! Ion signal from collision gas observed at eV c Lifetime 20  s or longer.

2) First cell RE (C 6 H 6 +, ) = eV IE (CS 2 ) =10.07 eV IE (CH 3 Cl) = eV Exoergic ! Ion signals from collision gas observed and can be identified. I I I I III I I I I I

3) Relative yield of collision gas ions vs. impact energy When A 2 E 2g state is fully quenched RE ( C 6 H 6 +, ) = eV IE, eV 1,3-C 4 H CS CH 3 Cl CH 3 F CH ~

When A 2 E 2g state is present RE ( C 6 H 6 +, ) = eV IE, eV 1,3-C 4 H CS CH 3 Cl CH 3 F CH ~

4. Summary  Collision gas ion yield is dramatically enhanced when the charge exchange is exoergic.  Detect charge exchange signal for various collision gases with different IE  Presence / absence of a very long –lived state. Estimation of its RE. Or, charge exchange  energy titration technique to probe excited electronic states.

IV. Benzene derivatives A. Halobenzenes 1e 1g 3b 1 1a 2 6b 2 2b 1 np C 6 H 6 C 6 H 5 X X  e - removal from 3b 1  (3b 1 ) -1 1a 2  (1a 2 ) -1 6b 2  (6b 2 ) -1 2b 1  (2b 1 ) -1 Hole states appearing in photoelectron spectra 6b 2 (Xnp ∥ character) 2b 1 (Xnp ⊥ character) J. Chem. Phys. In press, 2002.

 Widths of vibrational bands of & are comparable.  Possibility of very long lifetime for of C 6 H 5 Cl + C 6 H 5 Cl Photoelectron Spectrum

 Widths of vibrational bands of & are comparable.  Possibility of very long lifetime for of C 6 H 5 Br + C 6 H 5 Br Photoelectron Spectrum

 bands broader than  Rapid relaxation of of C 6 H 5 I + C 6 H 5 I Photoelectron Spectrum

(F2p ∥ ) -1  (F2p ∥ ) -1 bands broader than  Rapid relaxation C 6 H 5 F Photoelectron Spectrum

B. Triple bonds  6b 2   (C  X ∥ ) character 2b 1   (C  X ⊥ ) character  e - removal from 3b 1  1a 2  6b 2  1e 1g 3b 1 1a 2 6b 2 2b 1  C 6 H 6 C 6 H 5 CN/ C 6 H 5 C  CH C  X Hole states appearing in photoelectron spectra

Sharp vibrational bands for states.  Possibility of very long-lived states of C 6 H 5 CN +, C 6 H 5 C  CH +. C 6 H 5 CCH Photoelectron Spectrum C 6 H 5 CN Photoelectron Spectrum

C. Experimental results 1) C 6 H 5 Cl + RE (C 6 H 5 Cl +, ) = eV IE (CH 3 Cl) =11.28 eV  E = eV – eV = eV, ~ C 6 H 5 Cl + ( ) + CH 3 Cl  C 6 H 5 Cl + CH 3 Cl + CH 3 Cl + would be observed if B of C 6 H 5 Cl + is very long-lived. exoergic!

Partial mass spectrum of C 6 H 5 Cl generated by 20 eV EI recorded under the single focusing condition with 4006 eV acceleration energy is shown in (a). (b) and (c) are mass spectra in the same range recorded with CH 3 Cl in the collision cell floated at 3910 and 3960 V, respectively. Type II signals at m/z 49.3 and 50.3 in (b) and at m/z 49.6 and 50.6 in (c) are due to collision-induced dissociation of C 6 H 5 Cl +  to C 4 H 2 +  and C 4 H 3 +, respectively. The peaks at m/z 50.6 in (b) and at m/z 50.8 in (c) are due to collision- induced dissociation of C 6 H 5 + to C 4 H 3 +.

2) C 6 H 5 Br + RE (C 6 H 5 Br +, ) = eV IE (CH 3 Br) =10.54 eV  E = eV eV = eV, ~ C 6 H 5 Br + ( ) + CH 3 Br  C 6 H 5 Br + CH 3 Br + CH 3 Br + would be observed if B of C 6 H 5 Br + is very long-lived. exoergic!

Partial mass spectrum obtained under the single focusing condition with C 6 H 5 Br and CH 3 Br introduced into the ion source and collision cell, respectively. C 6 H 5 Br was ionized by 20 eV EI and acceleration energy was 4008 eV. Collision cell was floated at 3907 V.

3) C 6 H 5 CN + RE (C 6 H 5 CN +, ) = eV IE (CH 3 Cl) =11.28 eV  E = eV – eV = eV, ~ C 6 H 5 CN + ( ) + CH 3 Cl  C 6 H 5 CN + CH 3 Cl + CH 3 Cl + would be observed if B of C 6 H 5 CN + is very long-lived. exoergic!

Partial mass spectrum obtained under the single focusing condition with C 6 H 5 CN and CH 3 Cl introduced into the ion source and collision cell, respectively. C 6 H 5 CN was ionized by 20 eV EI and acceleration energy was 4007 eV. Collision cell was floated at 3910 V. Type II signals at m/z 49.3, 50.3, and 51.3 are due to collision-induced dissociation of C 6 H 5 CN +  to C 4 H 2 + , C 4 H 3 +, and C 4 H 4 + , respectively. Those at m/z 49.6 and 50.6 are due to collision-induced dissociation of C 6 H 4 +  to C 4 H 2 +  and C 4 H 3 +, respectively.

4) C 6 H 5 CCH + RE (C 6 H 5 CCH +, ) = eV IE (CS 2 ) =10.07 eV  E = eV eV = eV, ~ C 6 H 5 CCH + ( ) + CS 2  C 6 H 5 CCH + CS 2 + CS 2 + would be observed if B of C 6 H 5 CCH + is very long-lived. exoergic!

Partial mass spectrum obtained under the single focusing condition with C 6 H 5 CCH and CS 2 introduced into the ion source and collision cell, respectively. C 6 H 5 CCH was ionized by 14 eV EI and acceleration energy was 4006 eV. Collision cell was floated at 3942 V. Type II signals at m/z 73.5 and 75.7 are due to collision-induced dissociation of C 6 H 5 CCH +  to C 6 H 2 +  and C 6 H 4 + , respectively.

Collision gases, their ionization energies(IE) in eV, and success / failure to generate their ions by charge exchange with some precursor ions (CH 3 ) 2 CHNH O O O O 1,3-C 4 H O X O (butadiene) CS O CH 3 Br O O O X X C 2 H 5 Cl X CH 3 Cl O X O X C 2 H X O O X Xe X X X CHF X Precursor ion Collision gas IE, eV C 6 H 5 Cl + C 6 H 5 Br + C 6 H 5 CN + C 6 H 5 CCH + C 6 H 5 I + C 6 H 5 F + Recombination energy (X) Recombination energy (B) * ~ ~

Recombination energies of the X 2 B 1, A 2 A 2, and B 2 B 2 states and the oscillator strengths of the radiative transitions from the B 2 B 2 states. ~~ ~ ~ State C 6 H 5 CCH +  X 2 B ( ) A 2 A ( ) B 2 B Lowest quartet C 6 H 5 Cl +  ( ) ( ) C 6 H 5 Br +  ( ) ( ) C6H5I+C6H5I+ ( ) ( ) C 6 H 5 CN +  9.71 ( ) ( ) Reaction threshold ~ ~ ~  Radiative decay of B 2 B 2 is not efficient for all the cases.  B states are not dissociative.  The lowest quartet states lie ~2 eV above the B state. Relaxation by doublet – quartet intersystem crossing would not occur.  Internal conversion must be inefficient for the B states except for C 6 H 5 I +. For the B state of C 6 H 5 I +, internal conversion must be efficient. ~ ~ ~ ~ ~ 12.41

V. Vinyl derivatives A.Detection of Type III ions by double focusing mass spectrometry Type I :K I = eV S Type II :K II = e[V C + (m 2 /m 1 )(V S - V C )] Type III :K III = eV C VsVs Ion source Magnetic analyzer Electrostatic analyzer VcVc Collision cell Scheme 1. Set the electrostatic analyzer (kinetic energy analyzer) to transmit ions with kinetic energy eV c. 2. Scan the magnetic analyzer (momentum analyzer, or mass analyzer). Detect Type III ions only.

B. Vinyl halide  e - removal from a  (  C=C )  a (Xnp ∥ )  a  (Xnp ⊥ )  Hole states appearing in photoelectron spectra Xnp C 2 H 4 C 2 H 3 X X aa aa a  a ( Xnp ∥ character) a  ( Xnp ⊥ character)

1) Vinyl chloride  Sharp vibrational bands for  Possibility of very long lifetime.

2) Vinyl bromide  Sharp vibrational bands for  Possibility of very long lifetime.

3) Vinyl iodide  Sharp vibrational bands for  Possibility of very long lifetime.

C. CH 2 =CHCN, Acrylonitrile Possibility of very long lifetime for

D. CH 2 =CHF, Vinyl fluoride  Broad bands  Short lifetime for

E. Experimental results ~ 1) CH 2 =CHCl + RE ( CH 2 =CHCl +, ) = eV IE (CH 3 Cl) =11.28 eV  E = eV – eV = eV, ~ CH 2 =CHCl + ( ) + CH 3 Cl  CH 2 =CHCl + CH 3 Cl + CH 3 Cl + would be observed if A of CH 2 =CHCl + is very long-lived. exoergic!

A state of CH 2 =CHCl + is very long-lived. Single – focusing mass spectrum recorded for C 2 H 3 Cl with CH 3 Cl introduced to the first cell. Double – focusing mass spectrum ~ II

2) CH 2 =CHBr + RE ( CH 2 =CHBr +, ) = eV IE (CH 3 Br) =10.54 eV  E = eV – eV = eV, ~ CH 2 =CHBr + ( ) + CH 3 Br  CH 2 =CHBr + CH 3 Br + CH 3 Br + would be observed if A of CH 2 =CHBr + is very long-lived. exoergic!

A state of CH 2 =CHBr + is very long-lived. ~

3) CH 2 =CHI + RE ( CH 2 =CHI +, ) = eV IE ( allene : CH 2 =C=CH 2 ) =9.69 eV  E = 9.69 eV – eV = eV, ~ CH 2 =CHI + ( ) + CH 2 =C=CH 2  CH 2 =CHI + CH 2 =C=CH 2 + CH 2 =C=CH 2 + would be observed if A of CH 2 =CHI + is very long-lived. exoergic!

A state of CH 2 =CHI + is very long-lived. ~

4) CH 2 =CHCN + RE ( CH 2 =CHCN +, ) = eV IE (Xe) =12.12 eV  E = eV – eV = eV, ~ CH 2 =CHCN + ( ) + Xe  CH 2 =CHCN + Xe + Xe + would be observed if A of CH 2 =CHCN + is very long-lived. exoergic!

A state of CH 2 =CHCN + is very long-lived. ~

5) CH 2 =CHF + ~ CH 2 =CHF + ( ) + CH 3 F  CH 2 =CHF + CH 3 F + CH 3 F + would be observed if A of CH 2 =CHF + is very long-lived. exoergic! RE ( CH 2 =CHF +, ) = eV IE (CH 3 F) =12.50 eV  E = eV – eV = -1.3 eV,

A state of CH 2 =CHF + is not long-lived. ~ Mass spectrum of C 2 H 3 F generated by 20 eV EI recorded under the single focusing condition without CH 3 F. Mass spectrum of C 2 H 3 F generated by 20 eV EI recorded under the single focusing condition with CH 3 F.

Recombination energy (X) Recombination energy (A) Precursor ions Collision gas IE, eV C 2 H 3 Cl + C 2 H 3 Br + C 2 H 3 I + C 2 H 3 CN + C 2 H 3 F + ~ ~ X Ar X CH 3 F O X Xe XXO CH 3 Cl OXOO CH 3 Br O C 3 H 4 (Allene) OOOO ,3-C 4 H 6 (butadiene) X

VI. Conclusion 1.Charge exchange ionization has been developed as a useful technique to find very long-lived excited electronic states of polyatomic ions and estimate their recombination energies. 2.The following very long-lived excited electronic states have been found. C 6 H 6 +, CH 2 CHCl +, C 6 H 5 Cl +, CH 2 CHBr +, C 6 H 5 Br +, CH 2 CHI +, C 6 H 5 CN +, CH 2 CHCN +, C 6 H 5 CCH +, Much more than found over the past 50 years!