In Bohr's model of an atom, the formula used to find the energy between the 2 orbits and wavelength of emitted photon was valid only for single electron species like hydrogen.In the case of a multi-electron system like in the picture given above will the electron absorb a photon to go from 2s to 2p and also remitt a photon while dexciting from 2p to 2s.There are also elements like sulphur with two excited states thus showing variable covalency but how do the electrons not dexcite from higher energy orbital in a short time but give enough time gap to show two excitation states?Is the dexcitation and remission of photon a phenomenon which can only be seen when an electron goes from one shell to another like from n=1 to n=2 or can it also be seen when electron goes from orbitals and sub shells like 2s to 2p?Since there is an energy difference between the 2s and 2p sub shells there must be remission of photon on excitation but I did not find any online sources to verify this, so I need help.
The OP asked
Is the dexcitation and remission of photon a phenomenon which can only be seen when an electron goes from one shell to another like from n=1 to n=2 or can it also be seen when electron goes from orbitals and sub shells like 2s to 2p?
A simple answer can be provided by examining the 3s and 3p levels of sodium atoms. As is well known, adding a little table salt (sodium chloride) to a flame causes the flame to emit fairly intense yellow light that is characteristic and diagnostic of sodium. This photograph shows the sodium emission from my homemade alcohol burner, with the fuel (70% isopropanol/30% water) ‘salted’ with two pinches of salt.
In the flame, sodium ions get their electrons back and some of the resulting sodium atoms get excited by energetic collisions. This results in some sodium atoms having electrons in their 3p levels, as per the following simplified Grotrian diagram:
An excited sodium atom cannot stay excited indefinitely: it must de-excite and return to the ground state. The next figure shows that sodium de-excites from the 3p levels to the 3s level by emitting the famous yellow sodium D lines:
How do we know that the yellow light is actually two slightly different colors (wavelengths) of light, as in the above figure? The answer is that we perform spectroscopy on the light: we send the light to an instrument that disperses light, similar to how a simple prism or grating disperses sunlight into a rainbow of constituent colors.
To do the necessary spectroscopy, we first need a source of excited sodium atoms. There are numerous ways to get excited sodium atoms and a particularly useful and convenient method uses a sodium hollow cathode lamp (HCL). My photograph below shows a sodium and potassium HCL with neon fill gas. (The red color is intense and is due to the neon, similar to a neon light.) The photograph shows the light being collected by a lens, to focus it on the end of an optical fiber at the left. Note in the first figure that the optical fiber also appears just to the left of the flame. So the optical fiber can be used to collect light from the flame, as in the first figure, or from the HCL, as in the figure below.
The other end of the optical fiber serves to provide light as the input to a spectrometer or spectrograph. The next figure shows my homemade echelle spectrograph, with the optical fiber at the upper left.
Input light from the optical fiber is collimated and sent to the echelle grating. Diffracted light from the echelle grating is then cross-dispersed by the prism and the camera records the resulting spectrum, called an echellogram. The next photograph is a composite of two echellograms I obtained with the HCL setup shown in the previous figure.
The pair of sodium D lines are indicated in the echellogram. All the rest is mostly due to neon, with potassium also contributing some emission lines.
Finally, the next composite photograph compares the sodium D lines obtained from echellograms with HCL and flame emission sources. The D lines are narrowest in the HCL and are somewhat broadened in the flame.
Added echellograms, to illustrate how they show spectral content.
Echellogram of the sun: showing Fraunhofer lines in absorption, e.g., H alpha (the red Balmer line), sodium D lines, the magnesium triplet, etc. This is a continuum spectrum with missing dark spots due to absorption of specific wavelengths: the Fraunhofer lines.
White compact fluorescent lamp: showing mercury atomic emission lines and the red and green broad bands due to the rare earth-doped phosphors on the lamp’s glass. This is a continuum spectrum with some mercury atom spectral lines.
A longer exposure echellogram of the sodium and potassium HCL emission, showing the very bright (very over-exposed) red neon spectral lines. I suppressed them in the previous composite photo by using a blue filter. This is a discrete line spectrum due to sodium, potassium and neon. Maybe also impurities: the HCL is old.