welcome to the seventh lecture of this course entitled basics of fluorescence spectroscopy
till last class we discussed about the various aspect of absorption spectroscopy and today
our topic is that what is the fate of a excited state molecule right so what we have seen
in last six lectures that by absorption of radiation one can excite the molecule from
ground electronic level to the excited state of the excited electronic level that could
be the first excited state or second excited state and so on and so forth however my question
is now that after excitation after you promote the molecule from ground state to the excited
state what will be the fate of this molecule in the excited state whether the molecule
will remain in the excited state forever or the molecule will come back from the excited
state to the ground state right by deactive by some deactivation process right
so let us see this diagram over here as you know that after shining light the molecule
can promote from the ground state to the excited state and this could be at any different vibrational
levels like these are the different vibrational level of this excited state and that obviously
will be guided by the franck on principle right what is the nuclear position of the
ground electronic level and the excited state electronic level that we have already discussed
ok so now after you promote the molecule from the ground electronic level to the excited
electronic level or what will happen the molecule will eventually come back to the ground state
through several deactivation processes and that will going to discuss today
so after you excite the molecule right so to some excited vibrational level is usually
the molecule come back to the lowest vibrational level of the excited electronic state right
and that process is known as vibrational relaxation that process is known as vibrational relaxation
and that takes place within this ten to the power minus fourteen into ten to the minus
eleven second the in this time scale so once the molecule will come to the lowest vibrational
level of the excited electronic state then depending on the molecule depending on the
molecule this molecule can come back to the ground state either in the radiative way or
via non radiative process those details will going to discuss later on
but let us consider that this excess energy that what is excess energy from here to here
either here or here or here it depends on the franck condon principle also because for
emission franck condon principle is also applicable like in absorption right so the molecule will
come back from the excited state to the ground state by emitting the excess energy here in
this case the excess energy is this much this is my excess energy so this is my excess energy
and these excess energy is emitted as light right these excess energy is emitted as light
so while exciting you are using this light the light energy is proportional to this from
here to here that is for your excitation right for emission you are getting the light having
energy this much right and these process usually takes place in this time region ten to the
power minus nanosecond nanosecond time region right this nanosecond time region ok so
so eventually this molecule will come back from the excited state the ground state and
the molecule will spend some time in this state the molecule will spend some time in
the excited state for some molecule these dissipation
of excess energy may be as heat may be as a not as light right
so i can also have these dissipation as heat so in this case the molecule will not going
to emit a different color of light here this color of the excitation color of this emission
are different right but in this case of non genetic transition this there will be no emission
of light because it's just a excess energy is emitted as a heat right so this process
is called the radiative transition
and these process is called the non radiative transition generally represented as k r and
k n r right ok so let me show you and after after it come back to the vibrational level
of this ground state it will eventually come back to the lowest vibration level of
the ground state because that is the boltzmann allowed level because the energy difference
between the two vibrational levels are much higher compared to the energy available at
the room temperature ok
so now what i wanted to show here is that whether this franck condon principle is
also applicable in case of deactivation of the excited state molecule as you remember
i i showed this diagram where the excitation of this harmonic potential very simplified
molecular picture harmonic potential is most overlap is like v equal to zero to v one this
is zero two so zero one condition is the most intense transition while the molecule will
come back because i said that although you put the molecule in v equal to one or v equal
to two guided by the franck condon principle eventually the molecule will come back to
the v equal to zero level right so that is the vibrational relaxation
that takes about the picosecond time scale right so it will come back to this level
so now this is my molecule this is my a function of this molecule so these will come from excited
state to the ground state by radiative transition let's say so here you see the overlap in this
case is most in case of g double prime equal to one right here g double prime equal to
one and say g double prime equal to two right so it also guided by the franck condon principle
right ok so that's what i here show the absorption and fluorescence spectra of this molecule
anthraeene here you see this this is the zero zero band right its less intense the zero
one band for the emission again zero zero band is less intense than the zero one band
right so now if i put a mirror over here if i just put a mirror over here this right
hand side looks like a mirror image of this left hand side so this is known as mirror
image right this is known as mirror image ok that i will going to discuss later before
that let me again come to here and let us define few terms here that as i said that
this is vibrational relaxation part this is emission and this is known as the fluorescence
the fluorescence takes little longer time about nanosecond right to come from the excited
state to the ground state and in the ground electronic state also there could be vibrational
relaxation and if you now excite your system from not from ground state to the first excited
state but from ground state to the second excited state right then again the system
usually come to the zero vibrational level of the first excited state right
so if you excite your system from here to here ultimately this system will come to these
this particular level and then the emission process will takes place right these all this
deactivation process is best represented by one diagram called jablonski diagram
right which is given by the jablonski so in this case this s zero is denoted as the ground
electronic level and by the absorption of radiation you can either excite the molecule
from s zero to s one the first excited state or s zero to s two which takes a very short
amount of time and then let's let's consider here that you are exciting molecule from
ground electronic level to the second excited electronic level and in that second excited
electronic level the molecule will come back eventually to the first excited level
and that particular process is known as internal conversion right internal conversion which
takes about a picosecond time and the system may come back to the ground state by emitting
radiation so this is the radiative type of transition and that is our fluorescence
that is our fluorescence
now the system may also as i said the i can also excite the system from s zero
to s one in this case that after vibrational relaxation that same fluorescence will going
to observe right same fluorescence will going to observe so as you can see here another
important which i will obviously going to discuss in detail later that another
interesting observation that whatever light you are using to excite your system from either
from s zero to s two or s zero to s one or the different levels in the s one the emission
right emission wavelength that means the emission energy from here to here that emission energy
remain same right remain same this is well known kashas rule which we are going to discuss
later ok so now obviously the system may come from
this s one to s zero without emission of n light right that is my non radiative path
so here is my k n r here is my k r right now depending on the spin multiplicity this excited
level can be designated as either singlet or triplet that's why it is written s right
this my singlet and here is my triplet that means the spin multiplicity is t right here
is my triplet
so in this case the system can also undergo a spring spring crossover from singlet to
triplet set right so from s one to t one right and that process is known as intersystem crossing
the inter system crossing takes place in around ten to the power minus eleven or
in the time scale intersystem crossing it depends on the molecule obviously whatever
i am writing this time constants are typical but obviously you can see the fluorescence
may be in the order of picosecond or internal conversion may be much much faster
like in the ten to the power minus thirteen or ten to the power minus fourteen second
intersystem crossing could be much slower but this is a overall overall time scale right
in general in general time scale so once the molecule will come change the
state from singlet to triplet then this triplet two triplet one like t one two s zero transition
right will also may takes place but here what we can see is that this is the spin forbidden
transition so usually this transition takes more time
right this transition takes more time and this is called the phosphorescence here the
time constant is about so here the time is about ten two the power minus sixteen second
right or one microsecond so the fluorescence is i said ten to the power typically ten to
the power minus nine second right and in this case it is ten to the power minus six second
so these time constants right this time constant that how long right on an average the molecule
will take to come back from the zero th vibrational level of the first singlet excited state to
the ground state right that is the characteristic of the molecule and in case of fluorescence
this time scale is about ten to the power minus nine second in case of phosphorescence
where it is from t one to s zero this time scale is about ten to the power minus six
second and this is just because this for the phosphorescence this transition is not
spin allowed right completely and in case of fluorescence it is between the same spin
multiplicity so from s one to s zero so the time is less right
so this is also one important parameter of the fluorescence which i will going to discuss
later so as said that there could be a non radiative process also so in this case
this is my k r and this is my k n r so k n r is non radiative that means the excess energy
is emitted not as light not as the radiation but as heat so this is called the non radiative
transitions ok so now let us just see a little more about this fluorescence of how things
happen so this is the modified or simplified jablonski diagram over here i have removed
most of this s two state and so on so in this case as i said that when you are exciting
your molecule right from the ground state to the excited state that means you are promoting
one electron from homo to lumo or homo to lumo plus one where depending on whether
this particular transition is allowed or not right
so so when is shine light on electron moves from homo to lumo with retaining is spin
orientation right so in this case the spin multiplicity does not change right so here
what we have that in the excited state the situation is like this and simply this
spin can this excited state molecule can come back to the ground state that that means the
excited electron from homo to lumo again come back from lumo to homo and then you will get
the characteristic fluorescence light out of the system
however so this is my this fluorescence right this fluorescence this is this is my fluorescence
over here so this is my fluorescence is what i am showing over here right however in the
excited state right that spin may flip right that the spin may flip and after flipping
of the spin now the spin multiplicity changes from singlet to triplet and then this transition
may takes place so that during the transition spin again flip right and that is the forbidden
transition and i will get these phosphorescence over here which takes more time than the fluorescence
right so here is my phosphorescence and this is over here right ok
now let us see some properties of the fluorescence as i already mentioned briefly but now let
us write it one by one so the first one first one i said that fluorescence generally occurs
from the lowest energy vibrational state of the first singlet excited state and that is
my kasha's rule so let me write over here one is my kasha kashas rule it says that the
fluorescence takes place from the zero th vibrational level of first electronic excited
state right fluorescence occurs from the zero th vibrational level of first singlet excited
state isn't it i already showed you it means something it means that ok over here whether
you are exciting here emission comes from here whether you are exciting here emission
comes from here you excite here emission comes from here same energy level right it means
that emission spectrum does not depends on the excitation wavelength right
so now let me write my second point over here second point over here that emission spectrum
does not depends on excitation wavelength so better i explain
you this [emmi/emission] emission what is emission spectrum then right so what is emission
spectra you know the spectra right this is the plot of absorbance versus wavelength right
so let me let me give you this spectrum once again so this is my absorption spectra
let's say this is my molecule present in my sample cell
and you are measuring the absorption here is your i zero here is your i for the particular
lambda as i have already said so then a is equal to log i zero by i for this particular
lambda so this is a function of lambda then right so a plot of a versus lambda will give
you the absorption spectra of the sample right so let's say this is how it looks like so
when you excite over here so you are exciting from the ground electronic state to the excite
first singlet excited state so this is my s zero s one transition right and in this
case this is my s zero s two transition right so if i now draw the molecular energy level
over here so here is my ground state some vibrational level this is my first excited
state some vibrational level this is my second excited state some vibrational level so this
is s zero this is s one this is s two right
so you excite from here to here so you get this one right let's say get lambda two you
exit from here to here you get this a lambda one so lambda two lambda one right s s there
were several vibration levels associated with each electronic level so these peaks are not
sharp as i said right in the atomic case we will get a sharp line but in this case we
will not get a sharp line right so this looks like a vibrational band right and this corresponding
maxima these are the maxima this actually corresponds to the each transition over
here right ok now you can excite this molecule either here or here either in the s two or
in the s one and then eventually the fluorescence will come right
so when the fluorescence will takes place from here that's a lowest vibration level
from here either at this location or at this vibrational level or at this vibrational level
because all are all somehow allowed right the franck condon factor may be different
but there will be some factor so it is possible that the fluorescence will also come from
zero vibrational level of the first singlet excited state to some different vibrational
level of the ground electronic state right
so the the color the the wavelength of this emitted light is not one right it could
be many it could be many so this is now if you somehow measure the intensity intensity
of emitted light versus wavelength then for different wavelength the intensity will be
the different and if you plot it what you will going to see the intensity is very low
very low that means no light is coming at this region probably here you will get some
maximum over here so this one is my let say lambda three so that means whether you excite
here or excite here these emission characteristic emission maxima is my lambda three so this
is a plot of intensity of emitted light versus wavelength is known as the emission spectrum
of this molecule so this is referred to my emission spectrum
emission spectrum of this molecule so this emission spectrum does not depends on excitation
wavelength whether you are exciting over here or over here this emission spectrum will remain
same this is obviously there is some exception but but this is a general rule right ok
so the third one is i e is again we will come automatically over here as you can see that
these absorption guided by the franck condon principle emission is also guided by the frank
condon principle but emission takes place from the zero vibrational level so that means
emission wavelength emission maxima is right shifted or in the lower energy compared to
the absorption maximum right and that is called the stokes shift right so i can write here
that emission takes place at the longer wavelength than the absorption right so emission maxima
is longer than absorption maxima and this is called stokes shift stokes shift right
fourth another property right so here is let me write it and then explain it
excitation spectra spectrum is same as absorption spectrum what
does it mean it mean that means that these absorption spectrum and these excitation spectrum
let me first explain what is excitation spectrum excitation spectra is that you measure you
monitor the emission at a particular wavelength right and then scan the excitation wavelength
so what i want to say is this exchange the color suppose you are sitting over here at
this particular lambda three and you are checking what is the intensity that's it however you
are changing the excitation wavelength earlier to get this emission spectra you have excited
the system right either at lambda one or lambda two but now what you are doing you are changing
this lambda continuously right
so in this case what you are doing you are doing that intensity at lambda three that's
what you are monitoring versus lambda excitation then the dependence or spectra whatever you
will get will be very similar to this absorption spectrum right you will get a peak at lambda
one you will get another peak at lambda two and so on so this is the fourth property that
excitation spectra is same as why it is like that and how however we can prove it that
we will going to see later and the fifth one the final one is five is emission spectrum
is mirror image of absorption or excitation because they are same
right that's what we have already seen right we have already discussed and with this
let me finish this lecture and in the next day we will continue from here
thank you very much