Asundi, R K

Padhye, M R

English

IACS Institutional Repository

THE NEAR ULTRAVIOEET SPECTRA OF BENZENE, PART 11 A  COMPARATIVE STUDY OF THE ABSORPTION. EMISSION AND FLUORESCENCE BANDSBy R. K. .ASUNDI and M. R. PA D H YR(Received for publicallon, Nov. s, ;9->S ) . .ABSTRACT. A compirative study is made ot the ultraviolet benzene bands recorded in absorption, emission and fluoresoence, as regards the di,stribution ot inten.sity and the development of vibrational levels. It is found that the intensity distribution in the emission .spectrum is .substantially diHerent from that in the ah.sorptioii spectrum at room temperature and appears to be governed bv the Tloltzmann factor in the excited state and the Franpk' Condon principle. The intensity di.stribution also differs from that of fluorescence bands in which the distribution is generally governed by the condition.s of excitation. The emission .spectrum gives data for a mote complete picture of the state of the molecule and the .selection rules governing the transition to which all th t three spectra are due.The region of the near ultraviolet absorption spectrum of benzene is between 2765X and aaiSS. and the corresponding emission spectrum lies, in. the region 2536I to 3180I. Hence there is a considerable stretch of region which is common to both the spectra.* The first bands of the E  series lie outside this'region whereas the first bands of the series .4 , B, C, D , G , H ,(X and lie in this region. Among them C'o, B"i and B% with their associated 160 quanta and /P’- i , a n d  f/o are common to both spectra. In- D system D \, DS are common with the associated 160 progressions aS also and /i"„ (c/. Table I).The most prominent bands involving vibration which are absent in the emission spectrum are zl"u (« = 0, i ,  2 . . .)• These bands are very strong in absorption, A'\ being the strongest. Similarly in C o  series, C o  is present and is very weak in emission but the higher members are absent. In the B”a series the members higher than B’ a and in G‘‘o scries members other than G*o and also the members of the E series lying in this region are absent in emission although they are present in absorption. The band corresponding to the excitation of 1178 c*ff vibration'is found in absoiption but Hot fn emission. The bands corresponding to i  - 1 transition of 849 vibration which are prominent in absorption in this region are not present in emission. The bands which are present in emission but ate not observed in absorption are* The analysis of emission bands is given in Part I, The re.sults of analysis of absorption bauds proposed by Spotter cf aland Radio and Heck, extended by Gat forth and Ingold with which our own analysis agrees arc utilised in this part. .24T able IDcvelopnient of Different Band Systems in Various Near Ultraviolet Electronic Spectra of Benzene (Comparative Studyl2 J3  K , Asttndi and M. fi. Padhy&Absorption Rmission f^luorescence Emission(Radle and TJeck) (present investigations) (Ingold Wilson) (Austin and Elack)4 0 g —4 ®*4 0 -1 - 43-1 4 « - i - 4 «.i4 0 . J - 4 * .( 4 0 .j, - 4 « . 2A»i-Aii 4 0 .3 -/13.JAOt-A^t 4 0 .i_.l3., /A \ - A \ 4 0 - 5- 4 3 . 1A% \, \\/jPj 1—4 0 * -2 -4 ®s-2~ 1 3^-. 0B«o-BOou u B»o-BOo B“o-B‘ofl0 ,-/ p . /jOj-B^iW Oj-fll*i<0s-W*jh0i-h\1BO*. 3 - 5 32-2B O j-j-fl* * -,COo-C*o (% C»o-C«„c:«,-Oi co_ -^c’ -j C0 . 1- C 3-1 C « - i- 4 ^ .i C»-irOj-COj, C«-a C0- 2- C 3 -* CO-j-CO-2 C-0.,2C \ - C \ C0- J - C 3-3 CO-3 - C C s ■ .C O .,-C O -3• CO-1-CO. 4 C0. 4-C * .-!C 'l - 1- C 5i 1VltTC-violet Spectta of BenzeneT able I ( c o n i d .)239A.bsorption (Radle atid Reck)Emission(present investigations) I^ luorescence (Ingold Wilson) Emission (Austin and Black)D % -D \ D \ - D \D \~ D \ DKi d »-i - d3_jI»»2 I>®-2 D®a DO-2-DKtD<>3~D\ DK3-VK3 D0.3- D *-3D%D\-ie \ - e \E \ - E \  E»-x £ ”3£ “2-.-£'2-10*0-o'o 0*0-0*0 «®0-“®0 0*0-a®o»®2 a*.j—o<-4 0** J —o*_j0*2 o*.j-o®_2 0® a—o®-3 o®.j—o®_2o®i-x~“'i- l“®l-2-“'l-3o*a-i“"2-3 */9«0 /3»0-fl®0 0>o-0\ 3®o-/*®o/9®-i-/8'-l00-2-0*.2 B®-a-3*-a$f>.3- 0^ -30\ i - 0 K i 0O..t-0^.,-a j8»''j-j9>,-a240Absorption (Radltf find Berk)6-«o-(;2or.\-G \GS-GU6'®,_J ■ >.".0> I/IV-WVA',0B'o®iJ'a"C’Vr.«PVD\oT abi.K I icontd’)JR. K . Asundi and M. F^dhyeKmiss'ion(present investigation)i”.-.G%H\v4V -ylVFluorescence (Ingold Wilson)IP -i-IP  j.0—a' *>y—<«v“'"l-1,0-a' *tt'S.2-1/5'o"-/8VK % - K \  K \ - K \  K«_,k «.j- k:>.2KniTM.sion(Austin and Rlack)'K \ - K \Ultra-violet spectra of BenzeneTabus I i c o n t d ,) \241A b s o r p tio n  E m is s io n  Ennoresccnrc(R a d le  a n d  B eck)! (p resen t iiiv estiga lio n ^ ) ( liig o id  Wilson)E m iss io n(Au-^ tin ami Black)520 + 3 X5T3 .. M “i-9 2 3  1045 + 2 >4513 (2)'^ '3047 + 2  ^513- 606 + 2 X 1001-1596+2 Xioov (2)*- II7S + 2 X l(*OI520 + 2 X 774 (2)>1 9231476 + 2 X 774,> 923-1596 + 2X 774,, 9231045 + 2 X 774 (2)* M M 923— 606 + 2 X 364- 1596+2 X 364 1476 + 2 X 364 -1178 + 2 X 364 + 1045-703+364 (2)*(530-671 Lift )— 6u6 —2 XO71-1178 +  2 X 513,1045-2 X671(icoi — loif) b \ i i )+  520 — 2 X iu lO— 1596—2 X loio— 3047 — 2 X i(»io(774-983 )530-2x985-15964-3x77,)-3047+2x774 (364-70363;;)— 6c6 —2 X 703 + 520 + 2 X364»+ 520-2x703 1596-2X703+ 3«47+2 >^ 364 ^ T h e se  d e n o ted  th e u u m b er o f h a n d s  in  th e 160 p ro g re ssio n .242Tabt,u I (c o n Id ) .R. K. Asundi and M. R. PadhyeAbsorption (Kadle ainl Ikck)Ktni^ sioii(present invc.stigatioji)- 8 4 9 + 5 8 0  (3) *“•3»<58b- 2 x 5 8 6 + 9 2 3  (3 ) *  -2X586’!'2 XCJ23 - 6 0 6 + 2  X 586 +  520  +  2 X 5 8 6  M M +923 ,» +2x923(f-i648 l)2u)■I- 520-3 X 1648 -600 “ 2 X 1648 J045-2X1648 (?“ ino 62k)-606-2 X 1 I JO - 1 5 9 6 - 2  X IJIO- 3 0 4 7 - 2  X m u( > -  1326  ivj,j)+520-2X1326-6o6 '”2 X 1326(null-totally synini degenerate vibiations)- 8 4 9 + 5 8 6U + 2 X 240 0-2X404 0 - 2  X 3080I^ 'Inore,sconce (Jngold Wilson)- 2 X 849  (5 ) *  - 2 X 8 4 9 - 9 9 2  - 8 4 9 + 5 8 6  -  6 0 6 - 2 6 4  n -99^0 + 2X240X^ niisaion (Austin and black)0 2x404 corresponding to AF = 2 of the frequency, thetx% + 2X24o and /3"o + 2x240 bands and the 160 progression of the and series ic f. Table I).This shows that th(j* intensity distribution in the two spectra is not tlic same. This can be clearly seen from the study of the relative intensities of the prominent bands c>f different series which are common to both the spectra. The relevant data are given in Table II, The intensities marked arc only* These denote the number of bauds in the i6o progression,Vlira-viotet Spectra bf Benieiie. 24Juseful for the study of the relative intensities of different band groups and of the relative intensity distribution amongst the bands in tha group. The intensities in the present experiments have been visually estimated and the values given are, therefore, very rough. The intensities marked by Radle and Heck seem to be more reliable since their method of estimation is more accurate.TAhUi IIwA% 75003abs32-5(Uc o * ^s420330HJ8 3 4(UCTi•^"(1c\C»-i*C.2D.gcC6:11 7^341dJ'CtyfS2/>%D\/)0_i3 -S55T3040/ Is.XJCISabs. Ic;.2*XiO4■ fecrXJrd8abs.The intensities (1) given for balKi^ s in absorption spectrum are from Radle and beck, (1940), They have given iixtensity 7500 to the strongest band Aq®.The intensities (1) given for bauds in emission spectrum are tbose observed in the present experiments (Asimdi and Tadlive, 1949) the intei siti'cs being relative to the /^ (^| the strongest band being 10,Making due allowance for these facts certain conclusions can be draWil from this Table. Thus,(1) In absorption A"u and C\, bauds are stronger than 1V\, and !)''» whereas in emis.sion B"u and D"o are definitely more intense than .d'’„ and C"„.(2) .^“-1 and C"_i decrease in intensity in absorption whereas they increase in intensity m emission. The positive members of these scries do not lie in the spectral region of the emission spectrum.(3) Taking either or as H"-, (since it is due to Fermi degeneracybetween 1596 and 606 + 992 = 1598, it is seen that ])°-i and b"_,decrease in intensity jnore rapidly in absorption than in emission.(4) and D°i have greater intensity than B"u and D''„ in absorption but are slightly weaker than B \  and D“o in emission.(5) For B'’a and D“a the intensity falls both in ahsorption and emission compared to B “i and B"i.(6) The band involving 1596 (cither a'\ or B“o) i.s relatively stronger in emission than in absorption.The bands B"o, D*o, and /3®„ lie to the long wave-length limit of the absorption spectrum and bands A''u and to the short244 JR. K . Asundi and M. R . Padhyewavelength limit of the emissioti speclriun. This cannot be a sufBcierit argument for the weakness of the bands in the corresponding spectri more so in the emission spectrum because the bauds D"a and which are still to the shorter weavelength side than A^u and bands come oiit with appreciable density.In the A  series the 520 frequency in the upper state is excited and transition takes place between this and the zero level of the ground state. In C  series the fundamental transition is 606 (ground)— 2 X520 (excited), ni Bf 6 06— 0 and in D ,  2 x6o6— 520. It is found that transition involving high frequencies of excited state and of low ground state are stronger in absorption than in emission { v i d e  Table JI). This is to be expected on the basis of the Bollzmauu factor. Similarly transitions involving frequencies of low excited states and high ground state are strongci in emission than absorption. T llis fact seems to agree well with Sponer’s 1^ 1939) hypothesis that in flourescenbe (hence also in emission) the 1-0 and o-i transitions of the vibration should occur with inverted intensities corresponding to the Boltzmann distribution in the upper state. The relative inlensitics of the groups on this basis should decrease in the following order B, a, /3\ D ,  /I, C .  This is also generally borne out by the data. An interesting observation is that /) which involves two quanta of 606 in the ground state is stronger than A  which involves no ground state frequency, although both contain one quantum of 520 in the excited state. This illustrates the operation of the Franck-Condou principle in addition to the Boltzmann equilibrium in the excited state. T his inversion of intensity m aybe even perhaps due partly to selTabsorplion although in the present experiments such a possibility is rather remote.W ith the superposition of the 992 frequency in the ground state, the intensity of all the four A ,  B, C, and D  scries decreases in absorption-^ Bui in emission the intensity increases in A  and 6 'series for the lirst quaiiliim of 992 and decreases for the second quantum ; while it decreases uniformly for ii and D  the decrease is not so rapid as in absorption. The superposition of the 923 quantum in the upper state raises the intensity of the B"o and To'* bands in absorption for the first quantum after which it falls. In etnision it falls uniform ly, This seems to agree with another observation by Sponer c i ,  a L  (3939) that in absorption the .second member of the 923 progression is the strongest while in emission it should be the second member of the 992 progression which should be by far the most outstanding in intensity. The agreement is not strictly complete since the intensities of B2i and 1)^1, are slightly less than of By“ and Do° in emission. In absorption, however, the agreement is uniformly satisfactory.1 uruing to fluorescence spectrum in relation to emission spectrum, we find that the region of both the spectra is the same.* The flourscence* The data of lugold et al are used to analyse these h:mds along the lines adopted for absorption and emission. The general trend oi analysis is indicated by Sponer ct al, a detailed analysis agrees with that of Garforth and lugoId,Ulita-iiiolet Spectra oj Benzene 245spectrum gives a large number of bands belonging to A series while the emission spectrum does not. On the other hand, the D series is more .promi­nent in emission than in fluorscence. The emission spectrum also sh ovV S two intense parallel systems of bauds due to Fermi resonance extending to high quanta of vibrations in the ground state whereas in fluorescence they arc limited to two quanta. Similarly the 3062 a[„ (the value of which has been found to be 3054 in present experiments) which is expected to be present in fluorescence is not unambiguously found. In the case of emission all the bands belonging to ( ’ ) series are due to the excitation of this frequency in the ground state. In emission a number of uon-totally symmetrical degene­rate and nondegenerate frequencies are excited. Such is not the ease in fluorescence.*The intensity distribution in the fluorescence spectrum is rather irregular. T his has been already pointed out by Sponer ei ai and they say "T h e  explanation of the relative intensities of the different band groups in the fluorescence spectrum is somewhat complicated. This is due to the fact that the bands are a result of mechanism of excitation, subsequent collision, and reabsorption. A s we should expect series is the strongest in the spectrum .”  Furthermore, the scale of intensity adopted by Ingold and W ilson (1936) seems to be different from that of Wilson (1936) and since for a comijarison of intensities in the emission and fluorescence spectra it will be necessary to consider ail the fluorescence bands, it is not possible or profit­able to discuss the relative intensity distributiou in the two spectra. However, it appears that the course of intensity-decrease in fluorescence follows the order B ,  « C ,  A ,  and D  while in emission bands the order is B, a /3 , I) ,  A  and C ,  « ^  being Fermi doublets practical of equally intensity and represent vibration mixtures.Thus experimentally it is found that the emission spectrum obtained and discussed in Pait I of this series (Asuiidi and Padhye, 1949) shows a great advantage over the high pressure fluorescence in the matter of excitation of the numerous ground level frequencies. It also gives a picture of inten-sity distribution since it is neither influenced by .selection rules nor by self-ab.sorption to a vei'y great extent. Ihe latter can be eliminated by the proper adjustment of the experimental conditions. The electrical method of excitation develops also the weaker bands which are not found in fluorescence and thus it gives a more complete picture of the state of the molecule and the selection rules governing such a transition.* It may be noted that even though the same dispersion and resolation was used m both the casci, the bauds in fluorescence naturally show a very open structure while in tbe present case they are far from being resolved.a—i7iaP—62 4 6 R . K ,  A siin d i arid  M . R .  PadU y^R E F E R K N C E SAsUnd and Padfaye (1945^1 M ature, 1S6, 368.„ „  (i949)» J o u r.  P h y s . ,  23, 199 .GArforth and Ingold  (1948), /. C , S . ,  A p ril.Trifiold and W ilson  (1936), /■  C h e n i .  S o c . ,  955.Rudle and Beck (1940), /  C h e n i .  P U y . ,  8, 507.iip ou cr, N ordheim , S k la r  and T eller (1939), J, C h e n i .  Phy.^  7 , 207. W ilson  (1936), /. C h c m .  S o c .  P . ,  1210.

The Near Ultra-Violet Spectra of Benzene, Part II A Comparative Study of the Absorption, Emission and Fluorescence Bands

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/1119/1/The%20near%20Ultra-violet%20Spectra%20of%20Benzene%20Part%20II_R%20K%20Asundi%20and%20M%20R%20Padhye.pdf

The Near Ultra-Violet Spectra of Benzene, Part II A Comparative Study of the Absorption, Emission and Fluorescence Bands

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository