  Monolayer transition metal dichalcogenides (TMDs) hold great promise for
future information processing applications utilizing a combination of electron
spin and valley pseudospin. This unique spin system has led to observation of
the valley Zeeman effect in neutral and charged excitonic resonances under
applied magnetic fields. However, reported values of the trion valley Zeeman
splitting remain highly inconsistent across studies. Here, we utilize high
quality hBN encapsulated monolayer WSe$_2$ to enable simultaneous measurement
of both intervalley and intravalley trion photoluminescence. We find the valley
Zeeman splitting of each trion state to be describable only by a combination of
three distinct g-factors, one arising from the exciton-like valley Zeeman
effect, the other two, trion specific, g-factors associated with recoil of the
excess electron. This complex picture goes significantly beyond the valley
Zeeman effect reported for neutral excitons, and eliminates the ambiguity
surrounding the magneto-optical response of trions in tungsten based TMD
monolayers.
