[JamieK]

Hello, I have a question regarding finding the rotational constant. On a homework problem we were asked to "calculate the value of the rotational constant (to the nearest 0.1 1/cm) based on the lcoation of the peaks closest to the center cap. Convert this to an energy value in Joules." The two peaks closes to the center (Q peak) have values of 2139.27 and 2146.95.

I think that I've hit a "conceptual" block of some sort. I know that Tj =J(J+1)B and that B is what I am looking for, but what I don't understand that if J is an energy level ... can't it be infinately high? I mean I know that J can equal 0,1,2,3,4... etc. but how do you know WHICH one it would be equal to so that B can be solved for. I understand that by the selection rules J can change by +/- 1 (and V only by +1), but by the time that you reach the Q peak how do you know which energy level that you're at?

Any help would be greatly appreciated. I did ask my professor about it, and his hint was that deltaT = (3/2)V - 1/2(V) - 2(B).  And if delta T is the change between R1 and P1 it would be equal to 7.68, and I figured out V in an earlier problem to be equal to 6.42X10^15 1/s ... but if I plug those in B comes out to be significantly LARGE (3.21x10^15) which seems to be too large to me ...
Thanks in advance!

[gregpawin]

Tj or as I've seen it e_j, is the energy of a rigid rotor.  It would seem that this is a spectroscopy based problem about diatomic gases but your naming conventions are a bit different... do you know more about this delta T and V and what they mean?  What book do you study from?

[Donaldson Tan]

hope this proves helpful.

J is the rotational energy level

[Mitch]

and whats J specifically? I guess I could look it up though.  

[Juan R.]

Hi, Mitch,

JamieK, i do not understand to you

you claim that "V" varies only by +1 and i think that "V" is the vibrational level, but after you say that "V" is of order of 10^15!!!

I think that you are mixing V which is the vibrational level with v which is the frecuency of the line/band.

On any case

v = B J (J+1)

and

B = v / J (J+1)

therefore B is of order of v and more small, since J is positive. If v in your problem is 6.42X10^15 1/s then B could be so large like (3.21x10^15).

Note: For the CO B is so large like 5.76 10^10 (1/s)