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Comment on confirmation of the 3s Rydberg assignment of fluorobenzene
\‘olume 77. number 3
CHEMICAL
PHYSICS
1 February
LETTERS
1951
COMMENT
COMMENT ON CONFIRMATION J PHILIS, A BOLOWNOS, Deporrtttettr
of Pitysrcs,
Utztl ersttv
OF THE 3s RYDSERC
ASSIGNMENT
OF FLUOROBENZENE
G 4NDRITSOPOULOS of ioattrtttzo
fooontrrrta, Greece
E PAN-I-OS Sctettce Rcsearcit Couttctl, Doresbtrrl~ Loboraror_v Daresbut-) . Wamngtotr WA4 4A D. UK and T D S HAMILTON Deparrtttott Recewed
oj Pl~pstcs, 5 September
Utrtwrsrt_! 1980,
of Manchester
Mattchester
UI final form 29 October
,\T13 9PL
UK
1980
The band system at 6 3 1 eV III fluorobenzene detected by MPI spectroscopy IS also detected m the conventional gnsphase photoabsorptlon spectrum This system which overlaps the benzenold B,, state dlwppears in nlrrogen mntrL\ and pure sohd FBZ spectra thus confiimmg Its R)dber_e nature
The existence of a dipole forbldden smglet transltlon m benzene lymg between the wetl kno~\n 13,” and E,, states has been reported m the recent past employmg techmques such as electron-Impact [! ,2] and multlphoton lonlsntlon (MPI) spectroscopy [3 41 The two most likely candidates considered have been the valence ET, state and the II = 3 member of the s-Rydberg series Such a “benzenold” state would be dipole allowed m a substituted benzene of appropriate symmetry In recent pubhcatlons [5,6] the first member of rhe 3s Rydberg series of fluorobenzene (FBZ) 1s reported at 6 3 1 eV The equwnlenc benzene series ISof E,, symmetry and IS not allowed m D6h symmetry In Czv symmetry however, thus state has a 5, symmetry allowed component, the trnnsltlon bemg polansed out of the =_v molecular plane It should thus be possible m prmclple to observe this state even m a convent:onal gas-phase photoabsorptlon experunent Gllbert and Sandorfy [7] have reported energy values for some peaks appearing In the region where absorption IS attnbuted to the B,, state but have not presented the whole spectrum in which addltlonal structure exists Ttus IS shown m fig. lb. The MPI 0 009-26
14/8 1/0000-0000/S
02 50 0 North-Holland
ENERGY
-
EV
Fig 1 The single photon absorption spectra of fluorobsnzene, vertxaI sohd bars g?ve the posltlon of peaks found by hIPI [5,6] (a) Pure sohd spectrum shifted by 0 179 eV to hher energy (b) Gas-phase absorption spectrum (c) Nltrogen matrix spectrum shifted by 0 14 eV to higher energy.
Pubhshmg
Company
627
Volume
77, number
CHEMICAL
3
PHYSICS
bands of refs 15 _6] (fig I, verrlcal sold bars) comcrde wtth some of the sharper peaks m the smgle photon spectrum On the basrs of thus mformntron alone tt would be rather uncertam to assign thus structure to a separate band system of Rydberg rather than valence character Another candldate could be the already mentroned and long-sought-after valence Ez, state of the parent benzene molecule whrch 1s also allowed m Czv symmetry XlPI expernnents favour the detection of Rydberg states because they are thought to Irve longer than the htghly ewtted valence states. in the SlPI spectrum of benzene [3 ] 3 srmrldr band system IS seen attrrbuted also to the 3s Rydberg state rather thJn valence E 7e Thus was contimted by therm4 lensmg rxperrm
LETTERS
1 February
1981
shift The values of e (extmctron coefficrent) are vahd only for the gas-phase spectrum (fig. lb and tig. 2) Regnrdmg the Ezg state we should state that the results we present here do not preclude Its existence m the same energy region Ezs may also be present but overlapped and masked by the Intense Et, state in fact the spectra of two other fluorobenzenes, symtetrn- and sym-trl-fluorobenzene, where Et, IS srgmficantly blue-shafted show an absorption band too strong to be a Rydberg state [9] Fmally a comment IS due regnrdmg the band at 6 56 eV m the gas-phase spectrum, tentatively asstgned [7] JS the Rydberg 3p serves member Thus bdnd was not observed m the XIPI spectrum [6] and eklsts n-r our rntrogen rnatnx spectrum of FBZ In fig 2 we comoare the E 1u band envelope n-r the matrrx wrth the gas phase The 6 86 eV peJh can be attrrbuted to the O-O orrgm of the practtcally degenerate Et, + A,, The sphttmg of thts transttron 1s not seen and the matn feature of the matnx spectrum 1s a vrbroruc structure analogous to the one for the benzene matrrx spectrum [IO] A progression of three peaks IS observed m FBZ wrth a spacmg of -850 cm-’ while m benzene the Interval IS 430 cm -* [IO] This vibratron is the alo totally symmetric wrth ground-state values 993”cm-t and 806 cm-t for BZ and FBZ respectwely [ 111 The expenmental arrangement for the gas-phase spectra has been described m ref [ 121 and for matrl\ and sohd spectra m ref [ 131 IVe wish to thank rhe authors oi refs [5,6] for useful dlscusslons One of us (A-B ) acknowledges the NatIonal Hellentc Resemh Foundatton (E I E.) for a research scholarshlp.
References J.P
Doermg,
J Chem
Phys. 67 (1977)
4065.71
(1979)
7_0
7
ENERGY
rug
75
+
I
E”
2 The E,,, - A,g absorption spectra of fluorobenzene, pas-phnsc (sohd hne) and Nz-marrLu (dotted) The true posltion of the latter IS 0 149 eV further to the red
628
D G Wdden and J Comer, J Phys B 13 (1980) 67-7 P hl Johnson, J Chem. Phys 62 (1975) 4563.64 (1976) 4143 A J Twarowskl and D S. Khger, Chem Phys 20 (1977)
259 I;. ICrogh-Jespersen, R P Rava and L Goodman, Chem. Phys Letters 64 (1979) 413. K. Krogh-Jespersen, R P. Rava and L. Goodman, Phys 47 (1980) 321.
Chem
Volume [7]
77, number
3
CHEMICAL
PHYSICS
R Gdbert and C. Sandorfy, Chem Phys Letters 9 (1971) 121 [8] V Vatda, hl B. Robtn and N A Kueblcr, Cltem Phys Letters 58 (I 978) 557. [Y J J Phtlis, E Pantos, G. Andritsopoulos, A Bolovmos and I loanntdou, Chem Phys Letters 77 (1981) 623 [IO] B Katz, 91 Birth, B SharTand J. Jortner. J Chem Phys 52 (1970) 88
I February
LETTERS [I I [12]
198 1
] V Eaton
and D Steele, J hfol Spectry 48 (!973) 446 E Pantos. J. Phtis and A Bolovu~os, J hlol Spectry 72 (1978) 36. [I 3) E. Pantos, A.%1 Taleb, T D S Hamilton and I H Xlunro. Xl01 Phss
‘-8 (1974)
1139
629
CHEMICAL
PHYSICS
1 February
LETTERS
1951
COMMENT
COMMENT ON CONFIRMATION J PHILIS, A BOLOWNOS, Deporrtttettr
of Pitysrcs,
Utztl ersttv
OF THE 3s RYDSERC
ASSIGNMENT
OF FLUOROBENZENE
G 4NDRITSOPOULOS of ioattrtttzo
fooontrrrta, Greece
E PAN-I-OS Sctettce Rcsearcit Couttctl, Doresbtrrl~ Loboraror_v Daresbut-) . Wamngtotr WA4 4A D. UK and T D S HAMILTON Deparrtttott Recewed
oj Pl~pstcs, 5 September
Utrtwrsrt_! 1980,
of Manchester
Mattchester
UI final form 29 October
,\T13 9PL
UK
1980
The band system at 6 3 1 eV III fluorobenzene detected by MPI spectroscopy IS also detected m the conventional gnsphase photoabsorptlon spectrum This system which overlaps the benzenold B,, state dlwppears in nlrrogen mntrL\ and pure sohd FBZ spectra thus confiimmg Its R)dber_e nature
The existence of a dipole forbldden smglet transltlon m benzene lymg between the wetl kno~\n 13,” and E,, states has been reported m the recent past employmg techmques such as electron-Impact [! ,2] and multlphoton lonlsntlon (MPI) spectroscopy [3 41 The two most likely candidates considered have been the valence ET, state and the II = 3 member of the s-Rydberg series Such a “benzenold” state would be dipole allowed m a substituted benzene of appropriate symmetry In recent pubhcatlons [5,6] the first member of rhe 3s Rydberg series of fluorobenzene (FBZ) 1s reported at 6 3 1 eV The equwnlenc benzene series ISof E,, symmetry and IS not allowed m D6h symmetry In Czv symmetry however, thus state has a 5, symmetry allowed component, the trnnsltlon bemg polansed out of the =_v molecular plane It should thus be possible m prmclple to observe this state even m a convent:onal gas-phase photoabsorptlon experunent Gllbert and Sandorfy [7] have reported energy values for some peaks appearing In the region where absorption IS attnbuted to the B,, state but have not presented the whole spectrum in which addltlonal structure exists Ttus IS shown m fig. lb. The MPI 0 009-26
14/8 1/0000-0000/S
02 50 0 North-Holland
ENERGY
-
EV
Fig 1 The single photon absorption spectra of fluorobsnzene, vertxaI sohd bars g?ve the posltlon of peaks found by hIPI [5,6] (a) Pure sohd spectrum shifted by 0 179 eV to hher energy (b) Gas-phase absorption spectrum (c) Nltrogen matrix spectrum shifted by 0 14 eV to higher energy.
Pubhshmg
Company
627
Volume
77, number
CHEMICAL
3
PHYSICS
bands of refs 15 _6] (fig I, verrlcal sold bars) comcrde wtth some of the sharper peaks m the smgle photon spectrum On the basrs of thus mformntron alone tt would be rather uncertam to assign thus structure to a separate band system of Rydberg rather than valence character Another candldate could be the already mentroned and long-sought-after valence Ez, state of the parent benzene molecule whrch 1s also allowed m Czv symmetry XlPI expernnents favour the detection of Rydberg states because they are thought to Irve longer than the htghly ewtted valence states. in the SlPI spectrum of benzene [3 ] 3 srmrldr band system IS seen attrrbuted also to the 3s Rydberg state rather thJn valence E 7e Thus was contimted by therm4 lensmg rxperrm
LETTERS
1 February
1981
shift The values of e (extmctron coefficrent) are vahd only for the gas-phase spectrum (fig. lb and tig. 2) Regnrdmg the Ezg state we should state that the results we present here do not preclude Its existence m the same energy region Ezs may also be present but overlapped and masked by the Intense Et, state in fact the spectra of two other fluorobenzenes, symtetrn- and sym-trl-fluorobenzene, where Et, IS srgmficantly blue-shafted show an absorption band too strong to be a Rydberg state [9] Fmally a comment IS due regnrdmg the band at 6 56 eV m the gas-phase spectrum, tentatively asstgned [7] JS the Rydberg 3p serves member Thus bdnd was not observed m the XIPI spectrum [6] and eklsts n-r our rntrogen rnatnx spectrum of FBZ In fig 2 we comoare the E 1u band envelope n-r the matrrx wrth the gas phase The 6 86 eV peJh can be attrrbuted to the O-O orrgm of the practtcally degenerate Et, + A,, The sphttmg of thts transttron 1s not seen and the matn feature of the matnx spectrum 1s a vrbroruc structure analogous to the one for the benzene matrrx spectrum [IO] A progression of three peaks IS observed m FBZ wrth a spacmg of -850 cm-’ while m benzene the Interval IS 430 cm -* [IO] This vibratron is the alo totally symmetric wrth ground-state values 993”cm-t and 806 cm-t for BZ and FBZ respectwely [ 111 The expenmental arrangement for the gas-phase spectra has been described m ref [ 121 and for matrl\ and sohd spectra m ref [ 131 IVe wish to thank rhe authors oi refs [5,6] for useful dlscusslons One of us (A-B ) acknowledges the NatIonal Hellentc Resemh Foundatton (E I E.) for a research scholarshlp.
References J.P
Doermg,
J Chem
Phys. 67 (1977)
4065.71
(1979)
7_0
7
ENERGY
rug
75
+
I
E”
2 The E,,, - A,g absorption spectra of fluorobenzene, pas-phnsc (sohd hne) and Nz-marrLu (dotted) The true posltion of the latter IS 0 149 eV further to the red
628
D G Wdden and J Comer, J Phys B 13 (1980) 67-7 P hl Johnson, J Chem. Phys 62 (1975) 4563.64 (1976) 4143 A J Twarowskl and D S. Khger, Chem Phys 20 (1977)
259 I;. ICrogh-Jespersen, R P Rava and L Goodman, Chem. Phys Letters 64 (1979) 413. K. Krogh-Jespersen, R P. Rava and L. Goodman, Phys 47 (1980) 321.
Chem
Volume [7]
77, number
3
CHEMICAL
PHYSICS
R Gdbert and C. Sandorfy, Chem Phys Letters 9 (1971) 121 [8] V Vatda, hl B. Robtn and N A Kueblcr, Cltem Phys Letters 58 (I 978) 557. [Y J J Phtlis, E Pantos, G. Andritsopoulos, A Bolovmos and I loanntdou, Chem Phys Letters 77 (1981) 623 [IO] B Katz, 91 Birth, B SharTand J. Jortner. J Chem Phys 52 (1970) 88
I February
LETTERS [I I [12]
198 1
] V Eaton
and D Steele, J hfol Spectry 48 (!973) 446 E Pantos. J. Phtis and A Bolovu~os, J hlol Spectry 72 (1978) 36. [I 3) E. Pantos, A.%1 Taleb, T D S Hamilton and I H Xlunro. Xl01 Phss
‘-8 (1974)
1139
629