A photoelectron--photoion coincidence study of H2O

A photoelectron–photoion coincidence study of H2O, D2O, and (H2O)2 K. Norwood, A. Ali, and C. Y. Ng

Citation: J. Chem. Phys. 95, 8029 (1991); doi: 10.1063/1.461334 View online: https://doi.org/10.1063/1.461334 View Table of Contents: http://aip.scitation.org/toc/jcp/95/11 Published by the American Institute of Physics

Articles you may be interested in + Molecular beam photoelectron spectroscopy and femtosecond intramolecular dynamics of H2O and + D2O The Journal of Chemical Physics 85, 6928 (1986); 10.1063/1.451379 2 Dynamical study of nonadiabatic unimolecular reactions: The conical intersection between the B̃ B2 and 2 + Ã A1 states of H2O The Journal of Chemical Physics 78, 1246 (1983); 10.1063/1.444862 + 2 Infrared spectroscopy and equilibrium structure of H2O (X̃ B1) The Journal of Chemical Physics 97, 5977 (1992); 10.1063/1.463735 Time-of-Flight Mass Spectrometer with Improved Resolution Review of Scientific Instruments 26, 1150 (1955); 10.1063/1.1715212 + Photoionization of water clusters at 11.83 eV: Observation of unprotonated cluster ions (H2O) n (2≤n≤10) The Journal of Chemical Physics 84, 5561 (1986); 10.1063/1.449914 High-resolution photoionization spectrum of water molecules in a supersonic beam The Journal of Chemical Physics 88, 2249 (1988); 10.1063/1.454058

A photoelectron-photoion coincidence study of H20, 0 20, and (H 20)2 K. Norwood, A. AIi,a) and C. Y. Ng Ames Laboratory, b) U.S. Department 0/ Energy and Department o/Chemistry, Iowa State University, Ames, Iowa 50011 (Received 6 May 1991; accepted 14 August 1991) Photoelectron-photoion coincidence (PEPICO) data for OH + (OD + ), H + (D + ), and H 2 0 + (D 2 0 + ) from H;P (D 2 0) have been obtained in the region of 625-700 A. The PEPICO measurements allow the construction of breakdown diagrams for the unimolecular dissociation of energy-selected H 2 0 + and D 2 0 + in the Jj ZB2 state. The breakdown diagrams -2 for H 2 0 + (B B 2 ) ~nd D 2 0 + (B 2B2 ) in the internal energy range of 129-166 kcallmol are essentially identical. The branching ratios observed for H + (D + ) are higher than those reported previously. About 3%-5% of stable H 2 0 + (0 2 0+) is observed in the time scale of ~ 10 f.,ls. These stable H 20 + (0 2 0 + ) ions are attributed to ultrafast 73 2 B 2 ..... A 2A I nonradiative relaxation followed by the radiative stabilization from H 20 + (A 2A I ) --2 [D 2 0 + (A AI)] to H 2 0 + (X 2 B I )[D zO + (X 2B I )]. This observation also supports that the formation ofH + (0 +) via the HzO + (A 2A I ) [DzO+ (A01)] state is a viable process. The relative state-selected cross sections for the reaction H 20 + (X2B I ,AzA I; VI' V 2 ) + H20-+H30 + + H at center-of-mass collision energies" 1.9 eV have been examined using an effusive beam arrangement. Experimental evidence supports that vibrational and electronic excitations suppress the cross section of the latter reaction. We have also performed PEPICO measurements of the processes, (B 2 0h + hv-+ (H2 0)t + e- and H30+ + OH + e -, in the region of 1020-1110 A. The sums of the PEPICO intensities for H30 + and (H 20)2+ at corresponding photoionization wavelengths yield the photoelectron spectrum for (H zO)2'

I. INTRODUCTION

The water molecule, its clusters, and corresponding cations have been subjects of numerous experimental and theoretical studies owing to their great importance in solution, atmospheric, and interstellar chemistry. The water molecule in its electronic ground state has C 2u symmetry with the electronic configuration (la\)2 (2a 1)Z( Ib2 )2(3a z)2( Ib l )2. The first three electronic bands, X 2 B l' A 2A 1> and Jj 2B 2 , which correspond to the removal of an electron from the Ib l , 3a l , and Ib z molecular orbitals, are observable in the HeI photoelectron (PE) spectrum. l -4 The ground H 2 0 + (X 2B,) (PE) band is narrow and has the strongest (0,0,0) -> (0,0,0) vibrational peak, indicative of the nonbonding nature of the Ib l orbital. The vibrational manifold consists of the VI (symmetric stretch) and V 2 (symmetric bending) modes. The H 2 0 + cA 2A I) PE band, exhibiting an extended V 2 progression, is perturbed by strong vibronic coupling to the X 2B 1 state. Karlsson et al. 3 suggest that the H 20 + cA 2A I ) state is nearly linear. The emission spectrum corresponding to the transition A zA 1 -+X 2 B 1 has been observed. 5•6 The radiative lifetimes of H 2 0 + and D 2 0 + in the vibrational levels of the A 2A I state have been Permanent address: Astrochemistry Branch, Code No. 691, Goddard Space Flight Center, NASA, Greenbelt, Maryland 20771. b) Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This article was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences. 0)

J, Chern. Phys. 95 (11), 1 December 1991

measured to be ~ 1 f.,lS.7,8 The third H 2 0 + (Jj 2B2 ) PE band has a complex band profile and the observed vibrational features have been assigned to combination anharmonic progressions in VI and V 2 modes. No fluorescence has been reported for the H 2 0 + (73 ZB 2 ) ions. The nonradiative decay of excited H 20 + and DzO + in the B 2 B2 state, as manifested by the PE vibrational band shapes, including internal conversion to the A ZA I and X 2B 1 states and the unimolecular predissociation processes: H 2 0 + (73 2B 2 ) [DzO+

+ H(D),

(1)

+ OH(OD),

(2)

.... OH + (OD + ) .... H+ (D+)

(73 2B2 )]

has attracted many theoreticaI9 - 11 and experimental l 2--17 investigations. Using the HeI PEPICO method, Eland 12 has examined the formation of OD + and D + from energy-selected D 2 0 + (jj 2 B 2 ). The unimolecular decomposition of energy-selected H 2 0 + (73 2B2 ) has not been examined previously. Metastable decay for excited H 20 + leading to the formation of OH + has been observed in an electron impact ionization experiment. 17 However, direct observation of metastable or stable H 2 0 + (D 2 0 + ) in a well-controlled experiment has not been reported. In this report, we present the results of a detailed PEPICO study of H 20 and OzO, which provide insight into the unimolecular decomposition mechanisms of H20 + (B 2B2 ) and D20 + (jj 2B2 ).

0021-9606/91/238029-09$03.00

© 1991 American institute of Physics

8029

8030

Norwood, Ali, and Ng: Photoelectron-photoion coincidence

Using the PEPICO method, we have also examined the bimolecular and unimolecular proton transfer reactions (3) lind (4), respectively, H 20 + (X2Bl,~4ZAl;Vl,V2) + H 20-+H3 0+ + OH, (3) H 20 + (X 2B 1)·H20--H3 0+ +OH. (4) Energy-selected H 20 + (X 2B 1) • H 20 ions are prepared here by photoionization of (H20) 2 formed in a supersonic expansion.

II. EXPERIMENT

The experimental arrangement of the PEPICO apparatus has been described in detail previously.18-2o Briefly, the apparatus consists of a 3 m near-normal incidence vacuum ultraviolet (vuv) monochromator, a capillary discharge lamp, a vuv light detector, a molecular beam source, a quadrupole mass spectrometer (QMS) for ion detection, and an electron energy analyzer optimized for threshold PE detection. In PEPICO studies of reactions 0), (2), and (3), pure H 2 0 is introduced into the photoionization region by a quartz nozzle as an effusive beam. The backing pressure of H 2 0 is determined by the vapor pressure of H 2 0 at room temperature (:::::::296 K), and the flow of H 20 into the photoionization chamber is controlled by a needle valve. In order to produce a sufficiently high intensity of (H2 0)2 for the PEPICO study, a continuous beam of H 20 seeded in Ar is produced by supersonic expansion through a stainless steel nozzle at :::::::350 K and a total stagnation pressure of :::::::650 Torr. The partial pressure for H 2 0 is determined by the vapor pressure of water in a constant temperature bath at 340 K. The gas beam is skimmed by a conical skimmer before entering the photoionization region, which is maintained at a pressure of 650 A agree with those obtained by Eland and Berkowitz. 16 However, the decreasing trend of PIE data for OH + and OD + as a function of increasing photon energy at A. < 650 A observed in the latter experiment is contrary to the slightly increasing trend found here. Table I lists the relative ion intensities, l(ion), ion = H 2 0 + , OH + , H + , and 0 + , measured at A. = 630660 A. These measurements confirm the· previous finding that the intensities for OH + and H + are :::::::40% and 5% of that for H 2 0 + .13-15.21 The large amount of H 20 + observed at photon energies above the thresholds of reactions (1) and (2) corresponds predominantly to the formation of energetic electrons and H 2 0 + with internal energies below these dissociation thresholds. As indicated by the results of the

J. Chern. Phys., Vol. 95, No. 11, 1 December 1991


4

8031

(a)

..

!: c

!!2 c

2

6B6±1.5A

" .Q

"...

.Q

...

0

'"

zO

0

e b4

Z

:t:

a..

....

e le

H+

(b)

0+

(b)

:t:

"-

Q.

....

~

z

.!?

"-

666:1:1.5~

....

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0

...... .....

2

664:t1.5A

~~2~5----~6~40=------~~

l

670

g~25~----~6740~----65~5----~6=7~0----~6~8~5----~7~00

FIG.!. PIE spectra for (a) OH+ and (b) H+ from H 20 in the region of 625-700 A.

PEPICOmeasurements (see Sec. III A 2), H 2 0 + ions associated with threshold electrons are found to dissociate overwhelmingly according to reactions (I) and (2). Despite the fact that the photon energies used are greater than the thermochemical threshold of 18.7 eV for the process,

(5) the intensities for 0 + are nearly in the noise level. Based on the correlation diagram for H 2 0 + dissociation, reaction (5) involves predissociation by the b 4A2 state. The very low yield of 0 + by process (5) has been rationalized to be due to unfavorable surface crossing points with respect to the Franck-Condon zone. 10 The abundant formation of 0 + at photon energies> 23 eV is attributed to the 0 + + 2H channel via the (2a l ) - I stateY

FIG. 2. PIE spectra for (a) OD + and (b) D + from D 20 in the region of 625-700 A.

Figures 4(a)-4(c) show the PEPICO spectra for D 2 0 + ,OD + , and D + from D 2 0 in the region of 625-700 A. The threshold PE spectrum ofD2 0, shown in Fig. 4(a), appears to be more complex than that of H 20. Structure similar to that observed in the threshold PE spectrum is also evident in the PEPICO spectrum for OD + . The PEPICO intensity [I(PEPICO») for H 20 + (D 20 + ) drops abruptly as the photon energy is increased to the threshold of reaction (I), concomitant with the rapid rise in the PEPICO intensity of OH+ (OD+).

TABLE 1. Relative intensities, lOon), ion = H 20 + ,OH+ ,0+, and H+, observed in photoionization of H 2 0 at selected wavelengths (A).a

2. PEP/CO spectra for HzO+{DzO+),OH+(OD+), and H+(D+) from HzO(DzO)

The PEPICO spectra for H 20 + , OH + , and H + from H 20 in the region 625-700 A are depicted in Figs. 3(a)3 (c). The threshold PE spectrum of H 20 is included in Fig. 3 (a). Most of the PE peaks resolved in this photon energy region have been assigned to excitation of (VI'O,O), VI = 1-5, of the Jj 2 B2 state. 3,4 These vibrational features are also discernible in the PEPICO spectrum for OR + .

A(A)

l(H2 0+ )

I(OH+)

I(H+ )

1(0+ )

660 650 640 630

100.0 100.0 100.0

33.1 40.9 45.2

0.07 0.05

••• b

••• b

0.84 2.1 3.2 3.8c

... b

0.04c

each photoionization wavelength, [I(H,O + ) J is arbitrarily normalized to 100. bNot measured.

a At

C

Relative to [I(H20 + ) 1measured at 640 A.



8032 6

4 4

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(b)

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a.

OH+

.....

(b)

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w 2

a.

a.

S

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...

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WO

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w

a.

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eel

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OL-____ 625

~

_______ L_ _ _ _

640

655

~~

A

______

670

~

____

685

~

700

625

665

685

FIG. 3. (a) (D) Threshold PE spectrum for H 20 in the region of 625-700 + from H 20 in the region of 625-700 A. (b) and (e) PEPICOspectraforOH+ andH+, respectively, fromHzOin the region of 625-700 A.

FIG. 4. Ca) (D) Threshold PE spectrum for DzO in the region of 625-700 A; (D) PEPICOspectrum for D2 0 + from H2 0 in the region of625-700 A. (b) and (e) PEPICOspectraforOD+ and D +, respectively, from D 20 in the region of 625-700 A.

The PEPICO intensity for OH + (OD + ) begins to fall at the threshold of reaction (2) and continues to decrease as the photon energy is increased. An important observation of this experiment is that the PEPICO intensities for H 20 + and D 20 + remain finite at photon energies above the thresholds for reactions (1) and (2), suggesting that stable and/or metastable H 20 + and D 2 0 + ions are produced with lifetimes similar to the time scale of this experiment, i.e., the flight times of H 20 + and D 20 + (::::; 10 f.ls). The PEPICO intensity for H + CD + ) also increases rapidly as the photon energy is increased slightly above the threshold for reaction (2). The PEPICO intensity for H+

(D +) is substantially lower than thatofOH+ (OD +). The kinetic energy release for reaction (2) is expected to appear overwhelmingly as the kinetic energy of the light photofragments H+ (D +). The high kinetic energies for H+ (D +) formed at photon energies above the threshold of reaction (2) may lead to poor collection efficiency for H + (D + ).

A; ct.) PEPICO spectrum for H 20

3. Breakdown diagrams for the unlmolecular dissociation of H2 0+(8 2B2) and 0 20+(8 2B2 )

The intensity of threshold PE, I(TPE), detected at a given photon energy is proportional to the number of en-



ergy-selected H 2 0 + (D 2 0 + ). In order to construct the breakdown diagram, we have calculated the ratios, Y[PEPICO(ion)] = I [PEPICO(ion) ]lI(TPE), ion = H?O+ (D20+), OH+ (00+), andH+ (0+), at correspo~ding photoionization wavelengths. The l[PEPICO (ion) ] vs A plots, as shown in Figs. 5(a)-5(c) for the

8033

H 2 0 + system and in Figs. 6(a)-6(c) for the D 2 0 + system, represent the relative probabilities for the formation of product ions as a function of the internal energies for H 20 + (B 2B2 ) and 0 2 0 + (B 2B2 ) , respectively. The signalto-noise ratios of the original PIPECO data for H 2 0 + and D 2 0 + are relatively poor. The spectra shown in Figs. 3(a),

(al

,.....,

r-'I

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~0 U fri

100

0

q. 8, 0

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:50

ffiE:!.

I~

50

10

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fi

'§

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~

(b)

OH+

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,.....,

......

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P: ~

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(b)

0

2

B

2

0

U

ffi

ffi~

A. .......

I~

1-

0 625

640

655

1

670

685

700

FIG. 5. The variation of the ratios of Ca) l[PEPICO(HP+) = I [PEPICO(H,O+ )II(TPE), (b) l[PEPICO(OH+) I = I [PEPICO(OH+ )11l(TPE), and (e) I[PEPICO(H+) 1= I[PEPICO(H +) jll(TPE) in the region of 625-700 A. Here I[PEPICO(H20 +)], I[PEPICO(HO +)], and I[PEPICO(H+) I are the PEPICO intensities for H 20 + ,OH + ,and H + from H 20. I(TPE) represents the threshold PE intensity for H 20. The values for l[PEPICO(H,O + ) I in the region below the dissociation threshold ofH,O + (8 2 B2 ) have been normalized to 100.

l

685

FIG. 6. The variation of the ratios of (a) l[PEPICO(HP + ) I = I[PEPICO(D 20+)]!I(TPE), (b) I[PEPICO(OH+)] =I[PEPICO (OD+}]lI(TPE), and (e) I[PEPICO(D+)] = I [PEPICO (D+)]I I(TPE) in the region of 625-700 A. Here I[PEPICO(D 20+)], I[PEPICO(DO +)], and I [PEPICO(D +) I are the PEPICO intensities for D 20, + OD + , and D + from 0 20. I(TPE) represents the threshold PE intensity for 0 20. The values for I [PEPICO (0 20 + ) I in the region below

the dissociation threshold ofD20


+

CD 2B2 ) have been normalized to 100.

8034

Norwood. Ali. and Ng: Photoeiectron-photoion coincidence

4(a), 5ea), and 6(a) are three-point averages of the original spectra. Minor structures observed in Figs. 5(a) and 6(a) are due to the poor signal-to-noise ratios of the original PEPICO data and to the averaging effect. The values for I[PEPICO(HzO + )] and I[PEPICO(D 20 + )] have been normalized to 100 in the region below the dissociation thresholds of H 20 + (li 2B 2 ) and D 20 + (li 2B 2 ), respectively. The transmission factors for H 20 + and OH + (D 20 + and OD + ) through the QMS system should be nearly identical because of the similar masses of the two ions. Furthermore, as pointed out above, the kinetic energ~ carried by the OH+ (OD +) fragments is expected to be very small. Thus the energy release of the dissociation process (1) should have little effect on the OH + (OD +) ion collection efficiency. The observation that the values for I[PEPICO(OH + or OD + )] are nearly constant in the photon energy region between the thresholds for reactions (1) and (2) supports the latter expectation. Since only three product channels are important in this photon energy region, the sum, ~(H20+) = I[PEPICO(H20+ + OH+H+)] and ~(D20+) = I[PEPICO(D20 + +OD+ +D+)], respectively, should be constant in this energy region. However this condition is not fulfilled at photon energies above the threshold for the formation of H + (D + ). We expect that the mass discrimination effect in the detection of H 20 + (D 2 0 + ) and OH + (OD + ) is insignificant. Using the same experimental arrangement, we have measured the PEPICO spectra for CzH + formed in the dissociative photoionization ofCzH2' which also involves the ejection of a H.22 The breakdown diagram constructed for such a process is consistent with the conclusion that the detection efficiency for C2H + is essentially independent of photoionization wavelength in the range of 645-717 A. This experiment suggests that the small increase in kinetic energy of OH ~ (OD +) with increasing photon energy probably has little effect in the detection efficiency of OH + (D 20 + ). If little mass and energy discrimination effects exist for the detection of H 2 0 + (D 20+) and OH + (OD + ), we must conclude that the detection efficiency for H + (D +) decreases as the photon energy is increased above the threshold for reaction (2). Effort has been made without success to improve the collection efficiencies of H + (D + ) formed at higher photon energies by increasing the height of extraction pulse up to 500 eV and its width up to 0.2 f-Ls. ~ At photon energies below the threshold for reaction (2), the contributions ofI[PEPICO(H+)] to ~(H20+) and I[PEPICO(D+)] to ~(D20+) are zero. Using the relative PEPICO intensities for HzO+ (D 20+) and OH+ (OD+) measured at A, = 675 A and normalizing the value for I [PEPICO (H20+ + OH+) {f [PEPICO (DzO+ + OD+)]} to be 100 in the photon energy region below the threshold of reaction (1), we obtain the breakdown curves for H 20+ (DzO+) and OH+ (OD+), I[PEPICO (H20+) ]I~(H20+){1[PEPICO(DzO+) ]I~(D20+) and

I[PEPICO(OH+)]I~(HzO+)

U[PEPICO(OD+) ]I~(D20+)}, as shown in Figs. 7(a) and [7 (b) ]. The breakdown curves for H+ and D + indicat-

~

8.....

(a)

100

:>A 2Al nonradiative relaxation and that the formaof H+ (0 + ) from the excited tion HzO + cA zAl,v') [D 2 0+ (A 2A 1,v')] isa viable process. The occurrence of process (6) implies that the predissociation lifetimes of excited H 2 0 + (0 20 + ) leading to the formation of OR + (OD +) and H + (D +) are in the range of

750

850

950

FIG. 8. (a) (+ ) ThresholdPEspectrum!orHLO; (D) PEPICOspectrum forH30+ formed in the reaction H 2 0 + (X 2 B!,A 'A,;v"v4 ) + H 2 0 at near thermal collision energies. (b) Threshold PE of H 20 in the region of 650-

990 A.


8036


height (120 V) of the voltage pulse used to extract the H 20 + photoions, we estimate that photoions formed in the photoionizing region attain a maximum velocity of 6.4 X 105 cmls (kinetic energy = 3.8 eV) in traveling a distance of 0.03 cm. Since the half width of the H 20 effusive beam at the photoionizing region is ..;;0.3 cm, reaction (3) may take place in a time scale of ::::;05. f-ls. This calculation indicates that product H30 + ions observed are formed at center-ofmass collision energies 10 f.ls. Stable H 2 0 + (D 2 0+) may be attributed to lJ2B2 -A 2AJ nonradiative relaxation followed by the radiative process (6). The branching ratios for H + (D + ) observed here are substantially greater than those obtained in the HeI PEPICO study. The observation of stable H 20 + (D 20 +) and the high branching ratios for H + CD + ) suggest that the formation of H+ (D+) via the H 2 0 + (A 2A 1 ) [D 20+ (A 2A 1 )] state is a viable process. The relative state-selected cross section for reaction (3) has been examined using an effusive beam arrangement. At center-of-mass collision energies