EXPERIMENT 6 INTRODUCTION TO SPECTROSCOPY INTRODUCTION Much of what we know about the structures of atoms and molecules has been learned through experiments in which photons electromagnetic radiation visible light microwaves ultraviolet or infrared radiation radio waves etc are emitted or absorbed by the atoms or molecules The energy of a photon is related to its frequency and wavelength according to 6 1 where h is Planck s constant and c is the speed of light The energy of an emitted or absorbed photon corresponds to the change in energy the atom or molecule experiences 6 2 Whether photons are absorbed or emitted is correlated with the type of energy change the atom or molecule is undergoing Thus for example a molecule can be raised to an excited electronic state by absorbing a visible or ultraviolet photon A molecule already in an excited electronic state can return to the unexcited or ground state by emitting a visible or ultraviolet photon The energies of photons in this portion of the electromagnetic spectrum correspond to the differences between the ground and excited electronic states For changes in vibrational or rotational energy infrared and microwave photons respectively have energies corresponding to the differences between states Careful analysis of the details of the radiation absorbed or emitted as a function of wavelength the absorption or emission spectrum coupled with the formulation of physical models to interpret and explain them has provided a wealth of detailed information about atoms and molecules In addition to the structural information that can be gained studies involving the absorption and emission of electromagnetic radiation have proven to be extremely useful in other practical ways For example even without knowing why particular wavelengths are absorbed or emitted we can often use the observed spectra to identify the substances responsible This is particularly true in the infrared region for organic molecules where many vibrational spectra have been recorded and cataloged and can often serve as fingerprints to identify what is present In a similar way the specific wavelengths of visible and ultraviolet radiation emitted by atoms and ions in a flame or in an electrical discharge can provide an unambiguous means of identification In fact a number of elements were first discovered in this way when previously unknown emissions were observed Spectroscopic measurements are now routinely employed in the analysis of chemical samples While measurement of the wavelengths emitted or absorbed can provide a convenient means for qualitative analysis of samples i e what is present measurement of how much light is emitted or absorbed can be used for quantitative analysis i e how much of a substance is present Carrying out such measurements is sometimes referred to as spectrometry from spectrum measure rather than spectroscopy Although quantitative experiments can be performed using various regions of the electromagnetic spectrum one of the most useful is the visible portion sometimes in combination with the ultraviolet region In this experiment we will be working with visible electromagnetic radiation ordinary light chhvEphoton initialfinalphotonEEEE In order to study the emission and absorption of visible light we will make use of an instrument known as a spectrometer A simplified drawing of the main components of our spectrometer is shown in FIGURE 6 1 A spectrometer is an instrument that accomplishes two main tasks First it disperses or spreads out the light entering it into all of the wavelengths or colors present This can be done with either a prism or a diffraction grating Our spectrometer uses a grating Second it provides a signal proportional to the intensity of the light of each wavelength It does this by directing the dispersed light onto a detector which provides the electrical signal In our spectrometer the detector consists of an array of 2048 tiny diodes arranged in a straight line and positioned so that the dispersed light is spread from one end of the array to the other Therefore we actually have 2048 tiny individual detectors and each one has light of a slightly different wavelength or color falling on it mirrors blue red diode array detector grating light in Figure 6 1 Simplified Spectrometer Diagram Whatever type of experiment we are carrying out the signal from the spectrometer is always just a set of values one from each of the tiny diodes indicating the intensity of the light reaching them In actual practice we reduce the amount of data to be handled by averaging the signals from the diodes in adjacent pairs thereby obtaining 1024 values from the original 2048 The diodes are more sensitive to red light than blue light so signals in the blue end of the spectrum will be somewhat reduced compared with the red end However the signal for any wavelength is proportional to the intensity of the light of that wavelength Thus for example no matter what the color the signal will double if the intensity of the light of that color is doubled When we use the spectrometer to measure an emission spectrum we simply direct the light emitted by the sample gas in a discharge tube flame etc into the spectrometer The set of values we get from the spectrometer can then be examined to see what wavelengths of light were emitted We will only do qualitative emission experiments where only the wavelengths and not the intensities of the emitted light are important so the variation of detector sensitivity with wavelength will not affect its usefulness When we use the spectrometer to measure an absorption spectrum the situation is quite different We use a lamp to supply light of all wavelengths throughout the visible region and use the spectrometer to determine the extent to which light of each wavelength is absorbed The physical arrangement is shown in FIGURE 6 2 The sample is contained in a cuvet a small container with clear windows I0 is the intensity of the light incident on the sample as a function of wavelength The wavelength variation of I0 is determined by the characteristics of the lamp We use a tungsten halogen lamp for which the intensity goes through a maximum in the visible region I is the intensity of the light remaining after passing through the sample At wavelengths where the sample does not absorb I is equal to I0 Where the sample does absorb I is less than I0 We measure I as shown in FIGURE I0 I lamp spectrometer s a m p l e Figure 6 2 Measurement of