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Hi, good morning everybody, friends we are back here again with this rate processes.
So, we were in reaction dynamics; and we started with this collision processes, and we discussed
many other aspects, and last in in our last lecture, we have taken up this mechanism like,
one is harpoon mechanism, and we also discussed this rebound dynamics. Now, as a, you know,
as a recapitulation, now harpoon mechanism; what is this harpoon mechanism?
Now, this reaction has got large cross section, very large cross section; cross section and in generally forward scattered products
are obtained forward scattered products; and in the translation mode, low energy is released,
and generally your product is in internally excited state. So, product in the internally
excited form excited state, and it is in this case, what happens that for for this harpoon
mechanism, fairly heavy reactants, plucks the light atom from the other reactant, and
and a and basically the remaining part is acting as as if it is a spectator, it is it
cannot do anything; so and reaction occurs. So, basically you know, the reaction is like,
I have given example of K plus I 2 or maybe K plus b I two giving rise to K I plus I 2
sorry K I plus I, so it plucks iodine from this I 2. And this is one thing, I discussed
next, we talked about this rebound dynamics rebound dynamics; and we have taken the example
of K plus CH 3 I giving rise to K I plus CH 3, and in this case, in this particular case,
cross section is small, so cross section small and back scattering, we we get back scattered
product, so back scattering; and the energy is mostly in in translation form. So, that
we talked about you can have other examples, like fluorine plus hydrogen giving rise to
H F plus H 1, H F is in the vibrationally excited state.
Next, we will start with controlling reagents; controlling reagents reagents and and product
characterization; now now how to select means how to select that; so in the molecular beam,
you can you can… There is option to to you know, set the velocity, select the velocity,
so selection of velocity, so the thing is selection of velocity in the molecular beam
- molecular beam.
So, basically you know, here you see that your two molecular beams, they are intersecting
over here; and they are at 90 degree angle perpendicular, and the detector is you know,
oriented somewhere in between. So, it it can move in this way; so, angle - as a function
of angle, it can change; so that the detector can detect the product in various angles.
Now, the question is do you really means have to keep the the two beams at perpendicular
direction at an angle of 90 degree always? So, the thing is that that that I can change
actually; now there there are means, now the way is like that, one thing is that how to
select the velocity; I mean, this selection of velocity may be it is not the absolute
velocity, but it may be a relative velocity, relative velocity as well, relative velocity
as well.
Now how to how to select the speed, I mean velocity; now this can be this can be achieved
by chopper like here; you know, like like the beam shutter, you can have a chopper,
like chopper blade you know, you know like fan blade, so you can chop you know, certain
certain speeds like that; so it is moving - this chopper is moving this way, so, it
it selects a specific you know, velocities; like depending on speed speed like, the chopper
can be like this; it is a part, I i am showing just a part of that that so, so when molecular
beam is perpendicularly falling on this, this portion is there is no you know block, I mean
it is it is open, so molecules can pass through it; next when say this is rotating this way,
so next instant maybe maybe for some time, suppose it it has got the speed, some speed,
so that I mean and your molecular beam is somewhere over here, so this will continue
to keep you know, keep the molecular beam to go to the other side of this page, as long
as this block is coming over here. So, this molecular beam will be will be on
for this much of time, I mean the time required for this this part to come to here. So, it
will remain on and then off for this much of time, so depending on speed of the the
molecules, which are able to come here, can pass through and those which are not able
to able to come to this plain of your paper, will not be able to pass to the other side
of the paper. So, slower moving slower moving molecules you can select, I mean slower slower
moving molecules you can you can stop and you can allow only faster moving or or vice
versa depending on your necessity, whether you can… If you are if you are and you know,
rotating this this chopper at at some different speed at at some lower speed, then it will
allow allow lower speed to pass through, so this way you can select. So, that is one mechanism;
so it is it is a kind of you know like this, this is although, it is it is beam is completely
blocked by this, but if you can arrange means, if you can replace this with a with a like
this chopper, you can select different speeds that is one thing.
Next is, I mean so, so use use of a chopper use of chopper, may be if you if you change
the pressure difference, pressure pressure difference between the source and the and
you know, target. So, so that will also also change the change the velocity, so controlling
pressure, controlling the pressure; and third is introduction of seed, introduction of seed
that is it is mixed with some some seed beam, you can you can change it, I mean so there
is collision between the seed molecules with with your reactant, and then where the velocity
may change. Another, option is that you know, changing of the relative velocity; so changing
of the relative relative velocity, relative velocity that is changing the relative velocity,
you can changing the relative velocity you can do it.
So, like if you can change the angle between this beam, so this is one one path and suppose
this next one is oriented at an angle, which is not exactly 90 degree may be this way,
so that means that means in that case suppose this is your A, and so and say this is your
this is your this is your originally this was your B, so your relative speed is this,
C relative. In other case, say A is remaining like this, like here and your your relative
speed is now is now has changed like somewhere here, say if you keep the length fixed, so
this is your C primed relative; say this is your A, and say, this is your B, so you can
you can change the relative speed by by changing the angle between by changing the angle between
between the two two beams; that is that is the intersection angle, the angle of intersection
that is the angle originally, it angle was like 90 degree; originally the angle was like
90 degree; as as here 90 degree, but you can change the angle, you can change the angle.
So, that will change the relative relative velocity.
Now, next is the question of selection of internal states internal states. So, selection
of of internal states, internal states means various quantum states; so you can do that
by applying applying inhomogeneous homogeneous fields; that is inhomogeneous electric fields;
the thing is that the molecule in that case, molecules I mean, the your your reacting molecules,
the requirement is requirement is that your molecule should be your molecule should be
molecule should be polar that is dipole moment mu should be should not be equal to 0. So,
dipole moment that the that should have permanent dipole permanent dipole moment.
And, what you do is that so you have got say plus or minus plate over here, so these two
plates are oppositely charged, and so, what you do is basically, you know say this is
your minus plate, say this is your plus plate, so this this is so this from this side your
molecular beam is coming; molecular beam containing polar molecules, so what will happen that
this cold molecular beam, they will tend to be deflected this way, so if we if we view…
This is side view; if we view view from the other side, it will be something like, it
is like hexagonal alternate plus minus, plus minus alternate; minus, this is plus, this
is minus, this is plus, this is minus, this is plus, so molecules will pass through this
inhomogeneous you know, electric field; so you see that it is not homogeneously changing
from here, if you go from here to here it is plus; again it is minus. So, and again
it is it is plus minus so, it is it is changing constantly with with position. So, it is changing
constantly with position. So, and depending on the internal state of
the molecules, internal state means, whether it is you know, like different j state that
is rotation on states, so you know, you can select. So, basically you know the velocity
with which so, what you have to do? You have to focus like right, so focus is somewhere
over here. So, these molecules will be you know, will be attracted, and then again it
will go this way. So, basically if the field is not there, they will pass you know, in
a linear fashion; the moment it is it is you know, you you apply electric field, it is
bulging out. So, it becomes like a concaved kind of convex kind of thing; and then ultimately
again it will it will focus over here. Now, this focusing will depend, means where
it will focus? It will depend on the velocity that is this velocity so focusing; so it will
depend on velocity and also depend it will depend on the electric field strength strength
and magnitude of magnitude of mu; and actually depends on on the on the on mu dot E; basically,
mu dot E means, mu E cos theta, so component of dipole moment along the field, so it is
a dot product, so component of dipole moment along the field, it will depend.
And, you know separation, I mean, energy change as a result of this this interaction with
dipole moment and E is given by mu dot E; so originally energetically degenerated - degenerated
pulse are now not degenerate, as a result of this interaction, because of this interaction
there are no longer they will no longer remain degenerate, so there will be some separation
of energy levels; so energy levels are you know, degeneracy is lifted depending on its
on its you know, depending on the interaction of your dipole moment with E. Now, then the
thing is that next is you can select; so one option is your inhomogeneous electric field,
using inhomogeneous electric field, you can select internal states. Now, you can do that
you know, by by also by by Laser excitation, there is another means another means, which
is called the Laser excitation method.
So, internal states can be selected can be selected with the help of Laser excitation;
and high number density, that is high population is you know sometime hypo population density
of the excited species is sometime difficult in cross molecular beam experiment. So, in
that case although you know, we can select, but depending on laser intensity and also
frequency, but it is it is possible, internal states can be selected by means of by by using
laser excitation. Next is, how to control the reagent orientation; so controlling the
reagent orientation controlling reagent orientation. Now, another option is although, I i talked
about this inhomogeneous electric field, where you can depending on the interaction between
your your field and between between the dipole and the field; you can you can select the
different velocities or you know, different internally excited states. Next, is you can
apply a very weak electric field, so weak electric field like you know, say there is
a weak electric field, say in between, in between these two parallel plates, and a polar
molecule will be oriented in a definite fashion, and it should not be very strong field, it
is a weak electric field; so polar molecules will be oriented oriented appropriately.
So, by this way you can you can have the have the orientation of the reagent; another important
thing is that, because whenever you do laser excitation, so laser is basically you know,
strong electric field. So, the moment this electrical field is passing through your reaction
mixture the moment, it is passing through, what is happening that and and also laser
is a is a you know, polarized source of radiation; so polarized means its electric vector has
got a definite you know definite plane, so this electric vector is oscillating in a definite
plane; so the moment it is passing through your mixture, gas mixture, because we are
dealing with gaseous reaction and molecular beams; so, the polar molecules will be oriented
will be oriented means, the there is a there is a great chance of these those molecules
you know getting oriented with respect to the electric field vector of your laser source.
So, this way there is you know there is a chance of orientation, so if you use a weak
electric field, then you can then you can so for by weak electric field, you can you
can get oriented spaces, the thing is that only important point is that that for polar
molecules polar molecules of course, ofcourse, for polar molecules and this orientation you
know, is because of the stark effect so and this is because of stark effect; so first
order stark effect; and and this average angle between the field and the dipole moment is
basically you know, mu dot E cosine theta average, which is you know which goes as like
this, so KM divided by J into J plus 1, this way. So, this way you can you can select,
you know select rotational states, you know applying your first order stark effect, you
can select different different states, depending on depending on how much electric field you
are applying. So, depending on electric field, this perturbation is coming accordingly, so
they are for your selectively selectively you know, generating the perturbed state.
Next is we will have we will have a you know, an apparatus, which is taken from journal
of chemical physics 2001 edition volume 115, where this apparatus you know, gives you the
idea of how to you know, do this this collision experiments. So, here you see that you know,
it is a reaction between H and carbon monoxide; so it means, it is a collision between H and
carbon monoxide, so oriented oriented H. Now you see here you have got H 2 O and argon,
they are mixed and then it is passed through a high electric field, electric discharge.
So, that this H is generated, then this H through a skimmer and is passed through this
you know hexagonally oriented plates, alternatively plus minus, plus minus this way. So, it basically
is a selector, it is a selector; and so it generates one partner of course, ofcourse,
it is a state selected partner; and then you have got another partner over here. It is
also a pulsed valve, and then it is skimmer, and there you have got two beams, and laser
beam is here, and this is an orienting field you are applying.
So, what is happening that you are measuring here the light induced fluorescence from your
from your products, and there is a photomultiplier tube, which detects the light induced fluorescence
with corresponding optics is here, corresponding optical arrangement, so which detects light
induced fluorescence; and laser beam which you know excites the sample, and then you
accordingly get the laser induced fluorescence. So, so, basically, it is a it is a typical
apparatus, by which you can study, so it is schematic and it is taken from chemical physics,
journal of chemical physics. So, it is an example of means, how to study and using of
course, ofcourse, the laser induced fluorescence as the detection technique.
Next we will move onto next we will move onto you know, is there any stereochemistry? That
is whether whether some specific orientation is important for a reaction to take place.
So, for this let us take one example that rubidium plus CH 3 I giving rise to rubidium
iodide plus C H 3. Now, the point is that you have got the point is here that you have got your CH
3 like this; 3 hydrogen and 1 iodine; and you have got say rubidium, which is coming
say from this way; and you want to know you want to know whether whether you are getting
any R b I in the backward scattered direction, so to see whether see signal, when iodine
of CH 3 I is pointing towards rubidium beam that is important to know, that means that
means look for R b I in the backscattered direction, so that is in this in this direction,
so it is the backscattered direction. So, so, basically you know, so if you if you
measure that then it should be giving you the idea that whether this geometry or the
other geometry is important or not. And, whether there is barrier, I mean if if it is approaching
from this side, whether this, you are you are getting your product or it is approaching
from this side. So, whether is there any any any means, the reaction faces any barrier
with respect to with respect to the angle at which it is attacking your reactant. So,
angle of attack is important; so angle of attack means this is your line, so maybe if
it is coming this way, this way, so maybe this is your angle of attack. So, whether
you know, your your backscattered product is very much dependent on the or the or the
extent of your product formation is dependent on on the angle.
So if you want to know more on that you know details on that you can you can go through
this Levine's text, so that is the that is the basic idea that maybe maybe this if it
is attacking from this side or maybe from this side or maybe this side. So, amount of
backscattered product you are you are getting is more or less. So, that will give you the
idea, so and not all the reactions display strong Steric effect, because if it is approaching
this side, so it cannot access this iodine; so, there is a there is some you know, Steric
factor, so the reactions which proceed without any barrier in on the potential energy surface
are not generally generally you know, showing showing strong steric effect. Next, is if we would like to if we would like to you
know, so here we we have you know, given you the idea of the steric factor, and also told
that if it is barrier less then steric factor is less.
Next is product states product states, so we have to we we would like to you may need to need to measure need
to measure measure populations of various rotational vibrational state; rotation vibration states
that is characterized by j and v numbers; of course, in the products, and this provides
information on the you know, state resolved cross section means, which you know, cross
section of specific state resolved reactions, which states are more you know having having
more cross section than than the other. And, that is you know quantified by p v primed,
j primed, which is like which goes as sigma v primed j primed divided by sigma, where
cross section is the sigma; so this sigma is the total cross section, it is provided,
it is known, we can we can find out you know, this probability probability factor.
Now if the velocities are not particularly selected, but if if they you know if the velocities
are you know thermalized, I mean they are just thermalized, then the population provides
information on the state resolved reaction rate constants; and the population provides,
this population provides you know the information on the nature of the potential energy surface.
So, next is for the measurement; this molecular beam, since molecular beam, molecular beams
so it is giving you a platform of collision free environment, so platform for collision
free environments; collision free environments. So that means there is more scrambling of
internal states by by energy transfer, that is one good advantage of molecular beam and
but it is very difficult you know, very difficult to probe into into difficult to probe the
populations of the internal states of the internal states internal internal states from crossed molecular molecular beam experiment. So, it is it is
very difficult to find out these things; so, there there requires other techniques by which
maybe you should be able to you should be able to find out you should be able to find
out this internal state population.
So, other methods are like you know, one is infrared chemiluminescence. Infrared chemiluminescence
So, luminescence as a result of luminescence as a result of chemical reaction; luminescence
as a result of chemical reactions, so there are plenty of examples you will be finding
in any standard physical chemistry text like luminal; so if you if you react with H 2 O
2, so it you know luminous; so it is a cyclic hydrazite, so it will luminesce; so and it
is in solution. Now, in gas phase reaction like H plus C l 2 giving rise to giving rise
to H C l v primed, j primed plus c l, so this is your you know, rotationally and vibrationally
both excited; both rotationally and vibrationally excited; excited you know, system; and it
emits it emits emits in the IR region; so it emits in the IR region, so basically what
is happening that your hydrogen atoms, these are generated these are generated by by electric
discharge electric discharge, and because of this collision, because of this reaction
H C l is generated, gas phase H C l is generated in the rovibrationally excited; state rotation
vibration both way excited; excited in both sense, So rovibrationally excited meaning
rotationally plus vibrationally vibraionally excited; rotationally and vibrationally excited
and it luminesce. So and if you if you can record if you can
record the emission spectra, as a function of frequency or as a function of lambda, wavelength
you can you can indeed find out the population that is population of the internal states,
population of various v and j states; so in this case, what we have to do that you know
low pressure, in low pressure case so about 10 to the power minus 2 pascal pressure, and
which is low pressure will ensure that that mean free path is of the order of, which will
ensure that mean free path of the of the of mean free path of the process is of the order
of vessel length, length and H C l, what is happening that this excited, this is you can
write as H C l star; so H C l hits the wall sticks, and then it is deactivated before
any secondary collision. So, on IR emission you know, you have to you have to record in
in IR emission spectrometer. So this is one way so you have to you have
to ensure that low enough pressure is there, so that this mean free path is about the vessel
size, H C l hits the wall, and then sticks and then deactivated before any secondary
collision; deactivated means this it is emitting IR photons; so the thing is that it looks
like an an interesting piece of experiment, but the problem is that although in this case,
it is a nice example nice example that you are getting IR emission, which means when
plotted this IR emission, which when plotted as a function of you know you know emission
intensity, intensity as a function of lambda will get you important information on the
population of various various levels, but the problem is limitation is that not all
products, I mean like this are are IR emitters and of course,ofcourse,,,, there should be
very strong IR emitter, otherwise it will be a problem, because if it emits but, very
but, very weakly then this this will be of not that you know usage.
So, in this case limitation is like limitation of this infrared IR chemiluminescence method,
to look into your internal state population; so, it is you know, limitation is like you
need you need strongly emitting species emitting species in IR, regarding regarding status the status
of of the ground state not possible; so, so we do not get any information about the ground state, so this is another
another you know another problem, and first is the species should be strongly emitting
in the IR, otherwise otherwise infrared luminescence detector, I mean you know, detector is you
know, it will not be very useful. So you need a infrared detector and your species should
be strongly emitting in in IR but, the problem is since it is it is emitting from some excited,
I mean rovibrational state, so rovibrational excited state; so you do not get any information
about the population of the of your ground state.
So, what what is happening to the ground state you know, it is difficult to know on that
positive points, that one is you know, limitation is a negative negative points. So, positive
points it is you know relatively relatively relatively universal, it
is universal method. Next, is IR emission can be detected by time resolved FT-IR techniques.
So, by means of FT-IR - Time Resolved time resolved FT-IR method, one can detect detect
those detect those FT-IR photons IR photons and it is it is it is it is very sensitive
very sensitive sensitive method. Nowadays this FT-IR technique has been has
been employed, so so positive you know, of course, ofcourse,,,, it it is very it is a
very positive you know, thing that that this method at least give you although not for
all reactants, you can get information about this various levels like vibrationally and
rotationally excited levels that is a population on that, but still for some reactions like
the reaction, I am using over here like H plus C l 2 giving rise to H C l in the rovibrationally
excited level, and H C l which is which is formed in the rovibrationally excited level,
gives rise to infrared photon, and that gives you information on the on the population of
this various levels. So, that is why, in that respect it is it
is a successful thing that using infrared chemiluminescence that is luminescence as
a result of chemical reaction, you can find out the information of the various, I mean
information of the population of various levels; although it is limited, I mean limited means
like H F H C l these are useful, I mean in limited there are there are limited number
of infrared emitters, and of course, ofcourse, ground state information you know, it is it
is not possible, because we are we are probing the upper level, so from upper level to lower
level it is coming and because of this luminescence, we are we can think of I mean we can we can
calculate the population with that the upper level, how much it is populated; we can we
can you can find out. So, I mean as a function of wavelength, as
a function of wavelength we can if we can record the luminescence spectra, we can get
information means, which one is more populated, which one is less populated, I mean higher
level, which higher level is more populated, whether this higher level or that higher level
is more populated. If this higher level is more populated, so chemiluminescence from
this level will be more intense, is intensified compared to maybe this level. So, from that
I mean from the intensity distribution you can get the idea of the population of this
level. So, so, that is one one important technique
infrared chemiluminescence, we can probe the internal state population although cross from
cross molecular beam experiment, it is it is difficult difficult to find out, but from
this method we can we can find out, so that is one method. Another method is Laser Induced
Fluorescence method, so another method method, which is used which is used is – LIF - Lasar-
Induced Fluorescene, Laser Induced Fluorescence to probe into the internal state population
for for looking into internal population of the of internal states, so that we will take
up in the next piece of lecture. So, before we conclude today's talk, let us
have a quick recap that reagents state, selection in molecular beam, we have talked about and
in this case, by using chopper controlling the pressure, source pressure and also introduction
of seed can you help in selecting velocity of the of the reactants; and regarding relative
speed, relative velocity, you can change the angle between your angle between your two
crossed beams, so that you can change so you can change -consederative and you can internal
state you can select by introduction of the inhomogeneous magnetic, I mean electric field
generally there are six you know, I mean alternatively oriented, hexagonally oriented, plus minus
plus minus poles, and by means of laser excitation, you can also select certain you know, internal
states. By the use of your first order stark effect,
you can also choose specific rotationally excited level, I have given a example of typical
example, I mean example of a typical apparatus by which you can you can study collisions.
And, stereochemistry is an important factor for this reaction I mean in certain cases
barrier is very much dependent on the angle of collision and looking into the back scattered
product, you can see whether this process is is having large barrier or low barrier,
and generally when it is low barrier, then steric factor does not play that much of important
role. And, to look into the internal state population,
we have discussed one we have discussed in in short in in that you know how you can measure
the internal state population that method is you know, infrared looking into the infrared
chemiluminescence. So, that will help in, I mean that will give you idea on the population
of the of the upper state rather than the lower state that is the ground state. So,
that is all for today; so, in the next piece of lecture, we will start with a technique,
which is called the laser induced fluorescence technique, so till then have nice time thank
you.