1.10atomic spectra


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1.10atomic spectra

  2. 2. Atomic spectraThe simplest atomic spectrum is that obtained byexamining the light emission from alow-pressure hydrogen arc by means of a visual spectrometer.A characteristic series ofcoloured lines (the Balmer series) is observed in Figure COMPILED BY TANVEER AHMED 2
  3. 3. Balmer seriesthese arise from the fall of electronsdown the quantum levels of thehydrogen atom,each level being adequatelycharacterized for the presentdiscussion by the relevant principalquantum number (n).The electrons are initially promoted tothe excited levels (n > 1) by theelectrical discharge,andthe Balmer series of lines is producedby spontaneous emission of lightenergyof very characteristic frequencies orwavelengths as the electrons return from thehigher excited states to the secondenergy state (n = 2). COMPILED BY TANVEER AHMED 3
  4. 4. Lymen series ( UV ) – Paschen (IR ) –Pfund SeriesObservations of the emissionsoutside the visible range show other lineseries in the UV (the Lyman series)andin the near-IR (Paschen series)and far-IR (Pfund series).The energy transitions giving rise tothese spectral emissions are alsoillustrated in Figure COMPILED BY TANVEER AHMED 4
  5. 5. atomic emission spectra ofmore complex atoms such assodium andmercuryTo explain the atomic emission spectra of more complex atoms such assodium andmercuryit is necessary to label the states using symbolsrepresentative of three of the four quantum numberswhich characterise the electrons in an atom. COMPILED BY TANVEER AHMED 5
  6. 6. Thus the inclusionof the secondary quantum number ldefines s, p, d and f electrons(l = 0, 1, 2 and 3 respectively)while the inclusion of the spin quantumnumber s (= ±1/2) gives theoverall resultant spinindicated by the superscripts in the termsymbolsused to definethe ground and excited states of the atom. COMPILED BY TANVEER AHMED 6
  7. 7. These concepts are incorporated in theatomic energy level diagrams for sodiumand mercury (Figure 1.35),in which thewavelengths of the characteristic lines inthe emission spectra of these atomsThe ground state of the sodium atom(electronic configuration 2, 8, 1) arisesfrom the electron in the outer 3s atomicorbital,whilst that of mercury(electronic configuration 2, 8, 18, 32, 18, 2)arises from the spin-pairedelectrons in the outer 6s atomic orbital. COMPILED BY TANVEER AHMED 7
  8. 8. Atomic absorption spectroscopy results from the reverse transitions in atoms,inwhich the absorption of a quantum of radiation absorbed results in the promotion of the electron in the atomfrom the ground-state energy level to an upper energy level. COMPILED BY TANVEER AHMED 8
  9. 9. Sodium ATOMIC SPECTRUMThus atomic sodium shows strong absorption at 589.3 nm due tothe reverse 3s to 3p transition (and at 330 nm due to 3s to 4p transition).Atomic absorption spectroscopyhas become one of the major analytical tools for determiningtrace amounts of metals in solution.Atomic absorption is also responsible for the dark lines(the Fraunhofer lines) seen in the spectrum of the sun.The sodium atomic absorption line was thefourth in the dominant series of lines first observed by Fraunhoferand was labelled asline D; to this day the orange-yellow 589.3 nm line of sodium(actually a pair of lines at 589.0 and 589.6 nmdue to electron spin differences) is known as the sodium D line. COMPILED BY TANVEER AHMED 9
  10. 10. Electronic transitions in theHe–Ne laserThe principles involved in laser action weredescribed in section 1.5.5, the important characteristic being theformation of a relatively long-lived excited state(the metastable state),which allows stimulated emission to begenerated before spontaneousemission takes place. COMPILED BY TANVEER AHMED 10
  11. 11. HELLIUM-NEON METASTABLE In the He–Ne laser electrical excitation ‘pumps’ one of the 1s outer electrons in the helium atom to the higher-energy 1s 2s excited state, which then transfers the energy (by collision) to the approximately equi- energy metastable He (2p 5s) state From which the characteristic red 632.8 nm laser radiation is produced by the transition shown in Figure 1.36. COMPILED BY TANVEER AHMED 11
  12. 12. HELLIUM-NEON METASTABLE Fast deactivation processes from the terminal 3p level of the laser transition ensures that sufficient helium atoms are restored to the ground state ready to undergo excitation by energy transfer and hence maintain the laser beam to give a continuous output (possible with this particular type of laser). Other transitions are possible with the neon atom, but the design of the laser cavity ensures that only the 632.8 nm radiation appears in the output beam (through one of the end mirrors, which is partially transmitting to the extent of about 1%). COMPILED BY TANVEER AHMED 12
  13. 13. UV absorption in simple moleculesIn the hydrogen molecule,the simplest of all molecules, the two atoms are held togetherby a single bond formed by the two atomic electrons combining (with their spins paired)  to form a ground-state s molecular orbital.The promotion of one of theelectrons into the nearest excited state canbe induced by absorption of radiation Very low down in the vacuum UV, at about 108 nm(Figure 1.37). COMPILED BY TANVEER AHMED 13
  14. 14. The absorption occurs so low in the UV because of the significant energy difference between  the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).To obtain absorption in a more accessible region of theUV (i.e. above 200 nm) it is necessary to use organic molecules with double bonds  or containing heteroatoms such as oxygen, nitrogen or sulphur.For example,  ethene with its single double bond absorbs at about 180 nm,  but 1,3-butadiene and 1,3,5- hexatriene absorb at longer wavelengths  with increasing strength of absorption  as indicated by the values of their molar absorptivities, emax (Table 1.7). COMPILED BY TANVEER AHMED 14
  15. 15. Molecular orbitals for 1,3-butadieneinvolving the p-electron double bonds areshown in Figure 1.38,along with a simple energy diagram of the possible electronictransitions that produce absorption in the UV. COMPILED BY TANVEER AHMED 15
  16. 16. The HOMO to LUMO (p ® p*) transition leads to the longest-wavelength absorption band for butadiene quoted inÊ Table1.7. Extension of the conjugated (alternate single- and double-bonded)systemto four double bonds leads toabsorption just above 400 nm and a yellow colour; β-carotene, with eleven conjugated double bonds, isthe major orange component in carrotsand other vegetables,and one of the most important of the carotenoid plant pigments.Lycopene, which gives tomatoes their red colour, is another example of anatural carotenoid colouring matter. COMPILED BY TANVEER AHMED 16
  17. 17. The UV absorption characteristics of methanal (formaldehyde) illustratesthe important influence of the oxygen heteroatom.In the methanal molecule bonding and nonbonding electrons are both involved in the ground state (Figure 1.39),with the lowest-energy transition arisingfrom a weak absorption band at about 270 nmdue toexcitation of one of the nonbonding electrons into an antibonding p* orbital. COMPILED BY TANVEER AHMED 17
  19. 19. The schematic UV absorption spectrumshows two bands of significantly different absorptionintensities (note the logarithmic absorptivity scale),which is typical of simple carbonylcompounds.In the vapour phase or in solution in anonpolar solvent, the 270 nm band ofmethanal shows sub-band fine structure which is due to the simultaneous changes in electronic and vibrational structure.Such vibrational structure in UVand visible absorption bands can berepresented schematically in energy leveldiagrams(Figure 1.40). COMPILED BY TANVEER AHMED 19
  20. 20. Absorption spectra of aromaticcompounds and simple colorants The structure of benzene is often represented as  three pairs of conjugated p-bonds  in the hexagonal ring structure, with three of the six p-orbital states available  being occupied in the ground state by spin-paired electrons. The UV spectrum of benzene shows an intense absorption band near 200 nm with a weaker but characteristic band near 255 nm. COMPILED BY TANVEER AHMED 20
  21. 21. This ‘benzenoid’ absorption bandshowshighly characteristic vibrationalstructure,but this is absent in the phenolspectrum, in which the band appears atlonger wavelengths (bathochromic shift)and is of greater intensity.This effect is enhancedif the phenol is made alkaline so that theOH group ionises to O –(Figure1.41). COMPILED BY TANVEER AHMED 21
  22. 22. The bathochromic shiftand enhanced intensity has beenattributed to the electrondonatingcapabilities of the OH and O– groups.Such electron-donating effects of so calledauxochromic groupshave long been used in the synthesis ofdye and pigmentmolecules,which by definition have to absorb stronglyin the visible region. COMPILED BY TANVEER AHMED 22
  23. 23. Azobenzene absorbs weakly justbelow 400 nm,but substitution with an electrondonatingOH or NH2 group in the para position givesa simple disperse dye.Incorporationof both electron-donatingand electron-accepting groups(NO2 groups, for instance)at oppositeends of the azobenzene structuregives an intense orange dispersedye.The principle of incorporating donor–acceptor groups in the synthesis of dyes andpigments is widely applied and is wellillustrated in the anthraquinone series COMPILED BY TANVEER AHMED 23
  24. 24. Interaction with radiation duringphoton absorption causeselectron movementandcreates excited stateswith significantly higher dipoles than those in the ground-statemolecule. It is presumed that the donor–acceptor groups in dye and pigmentmoleculeshelp to stabilise the formation of the polarexcited states and hence result in stronglight absorption. COMPILED BY TANVEER AHMED 24