molecular spectroscopy Flashcards
what is the origin of spectra lines in molecular spectroscopy
absorption
emission
scattering of photons
+ the change of molecule energy
what changes can a molecule undergo in molecular spectroscopy
electronic transitions
rotational state
vibrational state
what does absorption of radiation do
transition occurs from a lower energy state to a higher energy state
what does emission do
transition from a high energy state to a low energy state
+ photon is also emitted
different types of emission spectroscopy
spontaneous
stimulated by radiation
absorption spectroscopy
absorption of radiation in order to transition to a higher energy state
general emission spectroscopy
molecule emits a photon in order to transition from a higher state to lower state.
what is spectroscopy
exchange of energy between light and matter
what can the different energy levels represent
vibrational energy
rotational energy
translational energy
electronic energy
photon energy difference between energy states/levels equation
🔺E = E2 - E1
where E2 is the final energy level
where E1 is the initial energy level
if photon energy = final energy - initial energy doesnt equal the gap
the energy will not be absorbed.
in absorption spec: when absorption is plotted against energy of photons
there is a peak
this peak is 🔺E
what is light and what does it have
wave
meaning it has a wavelength.
what is a wavelength
point from peak to peak or trough to trough
upside down extended v type shape
what is the wavelength range of visible light and what colours correspond to this
400nm —> 700nm
4️⃣0️⃣0️⃣. 🪓🔴🔴
frequency
sign and units
definition
v of f
number of wavelengths per second
s-1
Hz
speed of light
units sign
definition
c
2.998 x10^8 ms-1
2.998 x 10^10 cm s-1
speed of light = equation
speed of light = frequency x wavelength
c= v x 🌊
example: if blue light has a frequency of 6.4 x 10^14 Hz,,,,, what is it’s wavelength
c = v x 🌊
so 🌊 = c/v
2.998 x 10^10 / 6.4 x 10^14
= 4.68 x 10^-7 m
= 468nm
energy of light equation
units too girl
E = hv
energy = planks x frequency
J = Js x s-1
example: what is the energy of one photon of blue light when it’s frequency is 6.4 x 10^14 s-1
energy = planks constant x frequency
E= 6.626 x 10^-34 x 6.4 x 10^14
= 4.24 x 10^-19 J
how many photons does E = hv relate to
related to 1 photon
to get energy for a mole of photons multiply the answer by avogadros number
what is wave number
units too
v- = 1/🌊
m-1 or cm-1
how do we get from cm-1 to m-1
x 100
what is wave number proportional to
energy
frequency
frequency also relates to wave number and speed of light in what equation
v = c v-
how can wavenumber be fitted into the
E = hv equation
v = c v-
so E = hcv-
what is stimulated emission spectroscopy
who’s is when radiation is used to transition a molecule from a higher energy level to a lower energy level.
this leads to emission of a photon
what does the photo in emission spectroscopy have the same energy ( wavelength and frequency as)
same energy, frequency and wavelength as the incident photon
incident photon and emitted photon are in phase
what does the word laser stand for
light amplification by stimulated emissions
equation that connects energy, wavelength, planks constant and speed of light
E = hc / 🌊
light transmittance equation:
transmittance, T = I / I0
I = light before being absorbed
I0 = light transmitted aka the light that wasn’t absorbed.
what is the percent transmittance
(I / I0) x100 = %T
absorption equation
A = € x l x c
absorbable = molar extinction coefficient x concentration x length
what is € in the beer lambert equation
€ is the molar extinction coefficient
bc A = c x l x € what does this tell us about proportion
A is proportional to c and l
this is bc the proportion sign can be turned into an equals sign.
why does absorbable have no units
bc the units of the other stuff cancel out.
€ units = mol-1dm3 cm-1
c units = moldm-3
l units = cm
what is the relationship between absorbable and transmittance
A = -log (T)
A = -log ( I / I0 )
1dm is how many cm
10
1dm3 is how many cm
1000cm3
so conc in cm-3 x 100 given moldm-3
when beer lambert is plotted, € is given by the
the gradient / slope of the line
give 2 reasons as to why the beer lambert law fails when absorbance is plotted against concentration.
- at high conc: graph tails off as the molecules interact with one another and interfere with the absorbance
- at low conc : signal intensity is too low compared to the noise.
